I have mixed feelings about prompts. As a whole, I have far too many ideas in my head and fragments and notes about things I want to write to need to be prompted in order to have something to write. On the other hand, prompts can be fun. There's a lot of freedom in writing to a prompt that you didn't generate yourself, and therefore that you have nothing invested in. And there's a thrill to trying to come up with something on a topic you've never given an instant's thought to, on the spur of the moment. It's most interesting, in fact, when after the writing, you get together with a bunch of friends and see how a handful of writers grew completely different flowers out of the same seed.
So, thinking on it, I guess I don't really have mixed feelings at all-- a writing prompt can serve as a kick in the head, something to get you out of your rut, and I've gotten some good work out of prompted exercises. And even when I haven't gotten anything good, I've had fun.