I applied! It’s pretty selective, or at least according to previous year’s acceptances.
They usually have around 300 applicants (This year just under 300), and accept 60. However, not everyone who is a part of the initial 60 actually end up getting a professor to work with. Last year, only 52 people ended up being accepted. Not sure how it looks on college apps, but like most other programs, the worth of a summer program depends on what you take out of the program.
Also, another thread was created about this - check it out here!
http://talk.collegeconfidential.com/summer-programs/1714658-ucla-hssrp-2015.html