I'm not going to list the features for the SIP VOIP providers since they all pretty much include for free the same major feature set. (CallerID, Call Waiting etc.)
Vonage
My experience: (3 years)Vonage was my first VOIP provider. I had just moved (2003) and didn't want to keep paying the ridiculous international rates of the local phone service (Bellsouth). I would have changed earlier, but my DSL internet service at the previous address was unreliable (Bellsouth). I finally got my Adelphia cable internet service working after a month of phone calls trying to get them to make it work. I received a Motorola VT1000 adapter, plugged it in to a spare port on my router and it worked fine. Voice quality was as good as the old PSTN. I did experience an occassional outage here and there, but never longer than a day. After Hurricane Wilma, I ended up switching to DSL because Adelphia could not fix my cable internet after four months (Cell phone to the rescue). Vonage worked just fine with the DSL service. Eventually, I dropped Vonage because they were not keeping up with the competition's international rates. I was able to find international rates to my destinations that were 35% cheaper than Vonage. When I called to cancel, it took an hour on the phone and a threat to report them to the BBB if they didn't cancel my service.
Vonage Pros:
- Cheaper montly rates than Local Phone Service
- Free US long distance and lower international rates
- Included all the "extras" that the local phone company charges extra for
- Easy trouble free setup
- Reliable US calling
- International calls billed with monthly service
- Calling to my international destination was unreliable
- International rates not competitive anymore
- Difficult to cancel service
Vonage Spot Rating: 
I would have stayed if they had kept up with the international rates
and had better reliability to my destinations. They lost another spot due
to the cancelling annoyance.
Vonics Digital
My experience: (1 year)I decided I wanted to switch from Vonage and came across Vonics Digital which looked promising. I called them first to confirm that calling to my destination actually worked and the rates. They assured me that they used only "white routes" blahh blahh blahh and that it would work. I went ahead and signed up. I received an Innomedia MTA 6328-2Re adapter a week later. I swapped out the Vonage Motorola VT1000 adapter with the Innomedia MTA 6328-2Re. It worked fine. I made a few test calls in the U.S. and found the voice quality to be excellent. I attempted to make a call to my international destination and got a message like "You do not have enough credit to make this call". I got on their website and noticed that international calling required prepaid credit. Not a big deal, but less convenient than Vonage. I purchased $10.00 in credit. I made another attempt and got this message. "The destination you are calling is unreachable". I tried again the next day and it worked. Quality of the call was much better than Vonage was. The next day, I got the destination unreachable message again. I emailed customer service about it. They wanted the number I was calling so they could make some tests. The next day they said the problem was fixed. I looked at my history and saw that their test calls had taken some of my credit. I told them about it and they put the credit back. The destination unreachable message never went away completely, but it didn't become a problem. As time went on, I discovered that my voip service went out quite a bit - usually when I wanted to use it (Saturday mornings). After 10 months, I couldn't make any calls. I called their customer service number and they said they were having problems with their server and try later. The service came back 2 days later (unacceptable). Then about a month after that, something else happened. the service went down completely down. I went to the website - website was down. I called the customer service number (from cell phone) and got the number you are calling has been disconnected! I called my own number and got the same thing! Email went unanswered. All this took place right around the SunRocket collapse. I thought for sure that Vonics was gone too. A few days later, I got an email from a gmail account claiming the CLEC is having problems and they were working on the problem. (How did that effect their website????) The service was out for a week (way unacceptable!!!). Time to look elsewhere. I had no problem cancelling the service.
Vonics Digital Pros:
- Cheaper monthly rates than Local Phone Service
- Free U.S. long distance and lower international rates
- Included all the "extras" that the local phone company charges extra for
- Easy trouble free setup
- Calling to my international destination was more reliable that Vonage
- Excellent voice quality
- Poor service reliability
- International calls require prepayment
- Questionable company stability
Vonics Digital Spot Rating: 
Too bad the reliability of the service was so poor. They could have
been a good VOIP service. Maybe they have improved since then (haha). I
still receive periodic emails telling me to restart my adapter due to system upgrades
even though I haven't had their service for over a year.
BetaMax Voipbuster.com
My experience:
I used this service only for international calling.
Calls to my destination were cheaper and more reliable than Vonage and Vonics Digital combined.
Betamax has a bunch of different domains offering great rates to different destination groups.
You need to pick on that offers what you are looking for. Note that not all of them
support SIP devices.
BetaMax Voipbuster.com Pros:
- Low rates
- More reliable than two previous providers
- Works with SIP adapter
- Mobile rate to my destination is 5x higher than landline
- Requires a funky SMS text message to confirm your account (doesn't always work)
BetaMax Voipbuster.com Spot Rating: 
Good rates, service and reliability.
BetaMax 12voip.com
My experience:
I used this service only for international calling.
Calls to my destination were cheaper and more reliable than Vonage and Vonics Digital combined.
This service actually uses the same servers as voipbuster.
I signed up for this service because they were offering free calls to my destination.
After a month they discontinued free calls to my destination. The new rate was
.007/min where the cheapest provider anywhere was .04/min. I had no complaint about the change.
After 10 months of good trouble free service they increased the rate by 10 times.
So now it costs .07/min for the same destination. While still a good rate for my destination,
I can beat it (with one of their own services!) so I will stop using this one
when my credit is used up.
BetaMax 12voip.com Pros:
- Low rates
- More reliable than two previous "real" VOIP providers
- Works with SIP adapter
- Same reliable server as voipbuster.com
- Discontinued free calling to my destination after a month
- Increased the rate to my destination by 10 times
- Requires a funky SMS text message to confirm your account (doesn't always work)
BetaMax 12voip.com Spot Rating:
It was a good run while it lasted - I saved a lot with this service.
Quality is good. Reliability was better than my "real" VOIP providers.
Had the best rates available to my destination.
Axvoice
My experience: (2 weeks)
I signed up for "Pay As You Go" service with Axvoice from their website with the intent
to use it for international calling. Their international rates to my destinations are the best
I can find at this time. I was sent an email that I will be contacted prior to account activation.
They were not able to reach me at the number I had given so they sent me another
email to that effect. I replied with another contact number. I was called the next day
and my account was setup. I then received another email with a link to complete the activation process.
I clicked the link and it said it was active. I went to the website and it did not accept my login.
The email has another url and that one allows me to login in - a bit strange.
I set up my SPA3102's dial plan to use them as an outbound gateway for international calls.
Within a few minutes, it was working.
Axvoice Pros:
- Best rates to my destinations so far
- Sound quality is good with my SPA3102 adapter
- No monthly fee
- Activation a bit strange - If you sign up over a weekend, expect activation delays
Axvoice Spot Rating: 
I am currently using this service for outgoing international calls.
I will update my rating as I gain more experience with the service.
For the moment it's good.