I recently carried and returned a BlackBerry 8820 for my job. I did this on a temporary basis as a stipulation of my vacation, since not enough notice was given I had to be on-call for at least two of the five days. My experience overall was a positive one, I was able to be responsive and somewhat productive what out of the office.
I never finished this one. Long story short, I found myself checking my mail too often. I felt slightly leashed to work. The 8820 was too big for my liking, and the keys too small. Yet, through all that I was able to be away from work without worrying too much that I'd miss out on things. I could communicate still, and I was able to make a big contribution with only a small amount of my day.
I decided not to press for a BB through work, but to get one for personal use. Last weekend I did just that. I chose the Pearl because it's more phone-like yet it has larger buttons that are easier for me to use. I'm trying to work something out where I can get my work email on this phone without the attached expectations of having a work BB. That way I can use my free time to help my productivity without my boss (or her boss, more importantly) feeling that they command 24 hours per day of my time. We'll see how that goes.