i used Gamefly for about 6-8 months, and the service was okay. i just decided that the turnaround time wasn't worth the price, which i thought was fairly steep. i've now been using Gameznflix since the "big" thread, and i'm very impressed. the website layout and navigation is just left of horrible, but their selection is decent, and i've rented about 6 games so far (3 at a time), and the speed's been excellent; certainly faster than Gamefly. i only slightly miss the lack of GBA and DS rentals, but since i buy most portable games that i want anyway (since they're usually cheaper than console titles), it's not that bad. personally, i think that for the price, Gameznflix is fantastic. even if they ship half the speed of GF, which would be hard since GF was often quite slow, i'm still getting 3 games for a little over half the price of the 2-game plan at GF.
the way i see it, the price quite literally cannot be beat, and if they suck, just dump 'em! that's my plan, anyway. good luck in whatever you choose.