We've used Blockbuster for two months now. We really like it so far. It's a pretty quick turnaround (three to four days from when we mail the old to when we recieve the new), and we like the in store coupons. We haven't really had any problems, except when we received one that was cracked. All we had to do was report it (just clicked on "DVD was damaged/unplayable" and that was that.) They sent us another copy of the movie right away.
There didn't seem to be much to distinguish NetFlix from Blockbuster, so we just chose Blockbuster because it's cheaper and you get the free in store rentals.