If you get one, get the Curve. Best one out there for real.
I've had one for about a year now. Totally depends on your work and lifestyle. I basically need to be available to my clients 24/7 and they certainly fucking take advantage of that.
The BlackBerry comes down purely to how you use it. My cell phone is a separate phone - on purpose. I don't want to have to carry my bberry around all the time or I'll be doing work every second of every day. If you can learn to put it down at times, you win. Used like this, it's awesome. I can answer emails and get shit done while I am in the car or at the beach, whatever.
However, when people know you have one (and at this point all my clients just assume that everyone has one) they will expect you to answer emails quickly at all hours of the day.