I'm about to start subscribing to one of these. For however long it manages to hang around, I think Blockbuster has the best overall service.
I live pretty far from the Gamefly distribution centers, and they're known for long turnaround times. BB allows for in-store returns (speeding up mail deliveries) and the in-store exchanges allow you to essentially have two games for the price of the one-disc service plan.
It probably won't be around too much longer, but it seems like the best option for now.