It's absolutely worth it to me, but the cost is not a major factor in my life.  I LOVE the device.  It's great fun and a fantastically useful tool.  I have never had any issues with AT&T (though I know others have), and find that it works very very well as a phone, in addition to its extraordinary bag of tricks.  I am a convert and will likely always have one.  Everyone I know who has one feels the same way, and the only real quibbles I have heard were from people who live in areas like NYC, where AT&T coverage isn't stellar.
FWIW I think the device itself is very fairly priced for what it can do, but the monthly service is obviously not cheap.  (That is not fairly an iPhone criticism but rather a provider criticism.)  I don't find it excessively costly, but I hope that in time, as smartphones become the norm, the rates will go down.  It isn't enough to create a problem for me, but I know an $80+ cell phone bill is something that many find unpalatable.