I love GF, wish I'd picked them up a lot sooner. There are a lot of games I want to try but can't justify the $s on and this gives you a great way to play them. Once I got signed on I started tracking my game spending and its gone down considerably. A huge benefit is that all their used games come with all the codes unlike Gamestop and are usually a bit cheaper too.
The other obvious benefit is on short games. Their not worth $60 and may get forgotten if too much time passes.
One big complaint, preorders are not for limited editions or special versions. So buyer beware. I know for Borderlands I felt a little burnt. Only other complaint I have is with the q size, it stops at 50 but I took that opportunity to drop off crap games I didn't care about.
Speaking of q, you can add future games and track release dates, but those also count against your q.