I used wi-fi calling through T-Mobile with a blackberry curve about 5 years ago and it was always perfect. Zero issues at all. I think that it depends on the wi-fi signal and where you're connecting. I'd connect at home and work, and I never used any of my minutes really, because all of my calling was on wi-fi or to my wife's phone which was free between us. So I could go literally six months without using a phone minute.
I've been on AT&T for 2 years and it was AWFUL in California (just in case that's where you are), and then with Verizon for the last two years, and it was quite a bit better throughout the western US and significantly better in California. I've had all kinds of phones. If you like the way iPad and iPod are setup, then you won't dislike the iPhone. Those who smack it are those who like open source, and that's terrific. Some people like a phone to just be a good phone. For me, the blackberry blew everything else away in my experiences for many years. It just worked. I got my email, and I was able to make phone calls. Then the iPhone was eventually upgraded to the point where it pushed email and it made phone calls and I was able to send and receive all of my email. So for me... I had no reason to hack around and 'customize' my phone, and so I used an iPhone and am still using an iPhone 4 as we speak.
I will buy whatever makes the most sense for my next phone financially. I could care less about 'customizing' the thing. I want to make calls. I want to receive calls. I want to send email. I want to receive email. Maybe I'll get the new Motorola Droid. It looks like a nice phone and supposedly does all of those things.
As far as prepaid services go... we've had Virgin Mobile for our son, and it was a good service. For the last year (since he lost that phone) we've had Cricket wireless, and it does the job. Just check coverage maps first. His phone is like $25 a month for unlimited text, data, and 400 minutes of talking. We've never come close on the minutes, and it helps us keep track of him between school, home, and friend's houses. The prepaid services are just as reliable and user friendly as Verizon, AT&T, Tmo, and Sprint, and many times share the same network you've already used and had success on. I believe Straight Talk is on Verizon's network, and I believe Virgin is on Sprint's. You might want to double check and do a comparo though. Tmobile is actually GREAT in the town I used to live in, because they were originally Voicestream's test market and always put in the #1 A-Plus newest of new technology, and when Tmo took over, they kept that going, so it was awesome. Where I live now though, Tmo sucks, because it is just on the periphery of that amazing little hub they built for testing purposes.
R.I.P. Dad. I miss you. You will never be forgotten
1/12/39 - 8/7/08