I bought it because I'm planning to run a Doctor Who-like game and I needed some inspiration. 100 Planets provided it is abundance. I've yet to find a world listed in it that I couldn't use in one way or another. I've even found things that match themes I had already included in another form giving me greater a expanse of possibilities.
Like: Enough details to tickle the little grey cells into life without swamping them.
Dislike: That I want to use them all now but I should save some for other games!