Gamesasylum: I've mentioned TrueAchievements before, but since then the site has had a clean and tidy up and had new bits added. If you haven't been there, the site works out your true Xbox 360 Gamerscore based on how rare your acquired achievements are and how hard they actually are to obtain.
You can also see your best achievements - mine are for finishing Lost and Ninja Gaiden II. Going by the stats on the site only 30 percent of people who own Ninja Gaiden II managed to get this achievement. So in other words, not many people bothered to finish it.