I mean, after YC, the second best startup accelerator, by the "if I didn't get into YC, which accelerator would I consider" metric. (SendGrid still tips the scales for historical performance in favor of TechStars, I guess)
Since I've never heard my entrepreneur friends say stuff like that about 500 Startups ;-) and it's probably too soon to rank accelerators on performance, I went looking for something more scientific. This evaluation is the best I could find -- 500 Startups is #10: http://tech.co/reports/startup-accelerator-report-2012
Personally speaking from a Silicon Valley perspective (and having worked at a YC company before), 500 Startups is tied with TechStars on my list. I've never heard of any of the other accelerators on that list except AngelPad.