I've been using Ooma for almost 2 years now. I love not having a phone bill.
i paid about $200 for the set up. We only have one phone in our home, so I ended up selling off that extra hub (or whatever it's called...I really don't remember anymore) for a decent amount on amazon...lowering the cost a bit more :)
We just use a cheapie cordless phone we got from a local store. Our service has been nearly perfect. Probably better than any landline I've every had. There have only been a handful of days during the time I've had it where my service went a little wonky - like dropping calls, or I couldn't get a dial tone. That's really not terrible! The quality of calls is normal. I haven't had any complaints. Well, a couple times my mom said it sounded like I was in a fish bowl...I hung up, called her back and things were normal. I seem to have the most problems with her actually, so maybe it's her phone? I'm not sure.
Anyway, all in all, I'd never go back! I do not miss the phone bill and surprise charges! You can make any calls, any time, including 911 (which we HAVE, btw) and you don't have any fees. International calls are different, but we never make those calls...so I can't help you out there.
We have pretty slow "high speed" internet. We go through qwest (well, they just sold....hm...) and pay $24.99/mo for the 7mbps service. It doesn't slow down our connection at all. My internet has actually been down in the past - for a week or so, and my ooma service still works just fine. I'm not sure how *that* works, but I don't mind ;)
It's also nice being able to listen to your voicemails online. Although, there is no way to personalize your inbox message - at least not for the "free" ooma service - that might bug some people, but the savings make it worth it.
Hope that helps a bit.
If you are hesitant...do it. You wont regret it :D