I dumped PSTN somewhere around 2002, first went to vonage and shortly after to Asterisk + PSTN gateways. Over these 9 years I think I've developed an idea of the pros and cons of VOIP.
* Call quality, on average, has been very good. This probably depends mostly on one's ISP, but call quality is better than a cell phone which most people are OK with. I prefer PCMU since it's what the telcos use and is a simple (little processing overhead) and raw codec. Keep in mind that it's possible to use codecs with higher quality (HD in marketing speak) that what's on the PSTN.
* Reliability is OK, but I've had occasional problems with PSTN gateways not getting calls out and the occasional dropped calls, which I'm not always sure where the blame lies. My biggest headache has been NAT, mostly when trying to bridge calls with someone else behind NAT. I prefer to try to bridge calls directly to keep latency to a minimum.
* Cost and features ROCK! Keeping a DID (phone number) with Vitelity is just $1.99 per month. I love paying just for the minutes I use, typically between $0.0008 and $0.0016 per minute. And of course the feature list in nearly left to the imagination with Asterisk.
I have my network equipment and IP phones on a UPS and my ISP (cable company) keeps functioning when my power goes out so my phones still work. I could see this being a problem for the average Joe though. A straight DC solution would be nice and could be cheaper that using a UPS.