Boy, a lot of posts putting down the concept by ignoring the fundamental concept that the idea is NOT for everyone. OP says he doesn't need to make many calls, so 2000-voice-minutes-per-month guy and other are probably not good candidates for this option. Those who value pocket-ability enough to pay the much greater toll are also not candidates for this option.
Where it works is for those that don't need that much 24/7 phone (voice) service (the mostly text & data people especially). It works for those that need to be able to call back but not necessarily to be immediately on call for inbound calls (though it can receive inbound calls). It works for those who don't mind carrying the bigger form factor- who have a bigger pocket (like a suit pocket), backpack, laptop or other case. It works for those that want to carry the least load possible (so they don't need an iPhone and an iPad).
Most importantly though- for those where it does work- it offers a HUGE cost savings vs. an iPhone. $20/month times 12 months = $240/year. In my own case, I only need cellular data for this just a few months each year so my bill will end up at about $80 for a WHOLE YEAR! Wifi works well enough for the rest of the year. Do I feel like I'm missing anything vs. an iPhone? Not a thing (for my personal situation).
I appreciate the "must be able to make & receive calls anywhere at anytime", heavy voice users, pocket-ability and so on. If that's very important to you, this idea is obviously not for you. For me, it's not important enough to pay about $1,000/year for those benefits.
It's a very good solution if most of your "cell phone" use is texting & data with only some voice needs. When you do need voice, it can be as good as Skype (in fact, Skype is an app I tend to use to make outbound calls to landlines & mobile). Call quality is as good as Skype tends to be. Talkatone is quite good for inbound calls, texts, etc. too. If you'd rather pocket about $750 or more each year than give it to AT&T, Verizon, etc and this fits your needs, it's a great way to go.