I've been using a Blackberry Curve for a while and I'm been entirely happy with it. Great phone, great battery life, and no problems - save one. When I purchased it I also added a tethering plan as I often work remote or head to a coffee shop for a working break, and hate to rely on whereever I am to have secure wireless. The phone only works at EDGE speeds, but its good enough to check email and browse a few pages. Barely good enough! Also, the whole tethering thing seemed to be shaky, sometimes it worked, sometimes it didn't. Very frustrating to need a connection and not be able to get one.
After one aggravation too many I switched to an ATT USB Air Card (Sierra Wireless). It's 3G and it's much more usable. Sometimes it's a little flaky about being detected by the machine, but once detected has been very good. It's $60/month, plus I still have to pay $30 or so to get email on my phone, but overall worth it. Always exceptions though, today I'm at a decent sized airport and needed to work, could not get it to connect - had to pay $7.99 on wireless. Frustrating, but clearly a coverage or interference issue rather than it failing to work. Makes me wish for national WiMax!
I will say that as I've watched others with their IPhone I get jealous, the browser is nicer and bigger, and often I turn to one of those friends when we're to have them look up directions or get info we need. I can do it on my phone, but not nearly as elegantly. New phones from RIM do out soon, but I'll be surprised if they achieve the same elegance. Will I upgrade or switch to the iPhone? That's a clear case of I don't know yet.