I've been using Beeminder for about 3 weeks, and I finally fell off the road for my "writing" goal last night. I still have two other goals, so it's not all doom and gloom.
One thing I've learnt is that quantifying creative goals by time does not work well for me. I originally set up the goal because I wanted to write more, but it didn't really have the desired effect. I'd find myself rewriting things just to meet the time quota, which wasn't really productive.
I usually only write when I'm in the mood, and it can be a couple of days before inspiration hits. Forcing myself to write for 30 minutes every day seemed like a good idea at the time, but in the end just led to me scrambling for ideas and producing very little of value.
Still, Beeminder itself works really well. It's encouraged me to keep my email under control, and I've spent way more time working on personal projects than I used to. I'm still trying to come up with a good system for tracking personal & work goals, but I'll probably go with a points-based approach.