I recently did a one month trial for GameFly - a guy on a site I use needed a final referral for a site that gave you an Xbox 360 once you did enough offers or referred people who did offers. It was $8.95 for one month, a single game out at a time.
I only got to rent TWO games in that time. The first, Super Paper Mario, just wasn't for me, so I put it back in the mail a few days later - it took about 7 days for me (I live in MN, the nearest GF place was in PA) for the game to get to them and another game to get to me. So for me, no, turnaround wasn't that quick. When I had signed up and read the rules, they said they usually tried to ship to you when they received word from the USPS that the game was coming back - this really didn't happen.
In my opinion, GameFly isn't bad if you have time to commit to gaming and if you have the money, but for me it just wasn't worth it - I was glad to be done with it, because by the time I got my second game, I only had 8 days or so left in my trial.
I remember there once was a free trial for a month, but I personally am not aware of it anymore - the deal I was shown was $8.95 for the month, then $12.95 or so after for a few months. Normal rate is $15.95, as you said. For 2 games at a time, it's $22.95 - I'm sure you can rent from a game store for cheaper.
One thing I will say is GameFly does have some good deals (very few, though) if you want to keep the used games - and if you buy, they'll send you the manual/case - but when you rent, you just get the disk. I don't feel it was worth it, but it's totally up to you. I got paid $50 (no lie) to do the trial for the guy so he could get his Xbox 360 Elite off that site, so maybe if you knew someone who used a site like that, you could get them to pay for the $8.95 and get a free month of rentals, so they get a referral. Or if you want to use those sites, sign up and then do the GameFly trial - might be better than just flat-out going to GameFly and doing the trial... if you have friends that will do the trials as well and help you earn a gaming system, that is

.