I bought a Glock 17 so I can try it out in IDPA. Unfortunately, the ergonomics on Glocks suck for me. I have to shift my grip to hit the mag release and the slide release. That sloooows down mag changes to a crawl. So, I need to get an extended mag release and an extended slide release.
I bought my triggers (smooth face), extended slide releases, and 3.5 lb connectors from glockparts.com. They were all Glock factory parts as well. I haven't installed an extended mag release on either of mine at this point.
edit: I can't get glockparts.com to open right now, but the same parts at comparable prices are available at glockworld.com and topglock.com.