The only online rental service I've ever used was intothegame.com. That was back in Sept-Oct 2004 when they were in beta, so I got to use it free during that period. Beta testers were able to lock themselves in at a rate of $14.95/mo for 2 games out at a time. Their normal price is $13.95/mo for 1 game, $19.95/mo for 2 games, $24.95/mo for 3 games, and $29.95/mo for 4 games. They also have a referral program, if your friends join, you get $1 off your monthly fee for each month that your friends are a member (I think you can have up to $10 off maximum, unless they changed it).
Since they're located in CA and I'm in CA, games came pretty quick, usually within a day after they were shipped. For East Coasters, it probably took about 3-5 days for them to get their games. I only know this because beta testers had to post in the forums with general feedback, questions, game requests, etc. The service seemed pretty good, but the selection was fairly limited at the time since they were just starting out, but that's probably changed by now.
I've never used Gamefly before, but they've been around for a while, so they must be doing something right.

They're also in CA, like most online rental places seem to be, so I'm sure I'd probably get games from them just as fast as I got them from intothegame.com.