[Maro]

There are probably a lot more of these sites out there than the "big ones". I estimate in the tens, but possible 100+. I'd spend a few days enumerating and categorizing the competition in terms of approach taken, features, etc. learning what their Google footprint is, whether they're mentioned in blogs, what Alexa says about them, etc. This would give you a better picture about what your chances of success are. Without actually doing this research, I'd say it's a crowded segment, so your chances of succeeding are probably relatively low. I'd wait for a better idea / differentiator.

Good luck!