The only downside I have experienced through TaskRabbit is that they take a percentage of your pay for pretty much being the "middle man"...although they can't touch your tips, if someone wants to tip you for doing an amazing job (a nice loophole). It also seems to take more than the 2 days they say it will take to be paid (my experience is generally 4 to 5 days on average).
All in all, I give Taskrabbit a thumbs up. Go ahead and give it a go...make some extra cash and have fun while you're at it. Wanna share your experience? Let me know how it goes for you...I'm always interested.