I have one.
I have an absolute blast on it. For what I use the bike for, I can't imagine another bike that I'd enjoy more, at least not without spending at least double what I spent on the DRZ. It corners like nothing else out there. I ride it 5 miles to and from work, around town, and around the twisty Ozark roads around here, which is where it really excels. The great thing about this vs a sportbike is that you can have fun at 30mph, you don't have to do triple digits.
If you leave it stock you can get 65 mpg, or you can mod the hell out of it and still get 55 or so. Aftermarket is awesome since it shares so many parts with the now 9-year old DRZ400E and S models. You can get great info on mods at thumpertalk.com, in the DRZ subforum.
Another thing is bodywork - you can replace every piece of plastic on it for $140 total (not counting the tank). So if you wreck it, which at some point you probably will, it won't cost a fortune to fix, if you even want to. Try THAT on a sportbike.
My criticisms - if your riding is going to consist mostly of straight, flat highway cruising, there are better choices out there. Being a single cylinder it isn't the smoothest engine out there and the vibration and noise is tiresome if the engine RPM is staying constant for long periods. I changed my rear sprocket from the stock 41 to a 38 and it helped this immensely...but I still wouldn't ride it to Sturgis or something.