A few words in regard to the algoritm competition: you should give past competitions a try in order to get some practice. Your rating is determined by your performance in live challenges.

I'm not sure if employers look at it. Of course, if you get into the higher ratings, it's a nice thing to have on your resume. It also depends on the employer as well, I remember for instance one of the founders of LikeALittle was a TopCoder India champion, so for a startup like that, TopCoder experience might be something that they find interesting.