I have a blackjack as well, I've also had a Cingular 8125. I would highly recommend a SmartPhone, but it depends on what you're doing.
The internet is NOT quite like a PC... it is slower, and more latent. You cannot generally run flash (there are a couple browsers that have flash lite, but not by default). I use mine for push email, and it's the best thing ever, I never touch a PC for mail any more.