I use both but currently have BlockBuster on hold for awhile, debating whether I should cancel altogether. While occasionally BB does have a flick that I can't get through NetFlix (nothing much you'd notice unless you are as obsessed with oldies and rarities as I am), and the cost of keeping 3 at a time plans with both services is less than a 6 at a time plan with either, almost my entire queue at BB (of close to 500 DVDs) is perpetually tied up. And unlike NetFlix, if the top DVD in your queue isn't available, BlockBuster will just put everything on hold for three days before sending the next available film in the list (it's always the next available DVD that was available three days earlier, and a queue update after the fact will not alter the situation once you're stuck waiting).
So... I can't recommend BlockBuster and I don't understand why anyone would choose their service over NetFlix, given they have compared the two, at this time. Heck, even the digital download situation is miles better, with NetFlix offering unlimited free viewing of available digital movies while watching a single movie online through BlockBuster costs money above your subscription. The only thing BlockBuster has to offer is their brick and mortar stores, allowing you to get faster turnover on rentals if you happen to have one local to you (I don't).
Another cool thing about NetFlix is their community features -- once you have an account there you get recommendations based on your reviews, and in my experience it's been a very accurate system (smart enough even to recognize that I love Harold Lloyd but don't like Charlie Chaplin, for example). Blockbuster has stock ratings based on I don't know what, and doesn't even get much in the way of user reviews, so it's more pot luck if you aren't aware of what you're looking at beforehand.
Bet I've seen a lot more movies this year than you have, though ;)
I buy things I really like, or that may be hard to find down the line. I buy a lot of TV shows by the season, too. But a ~$17 USD monthly subscription to one of these major services can get you as many as thirty movies a month (if you're really quick about watching and returning or have a BlockBuster store local to you). Granted, reality isn't often that optimal, but it's still waaaay cheaper than buying every movie you see.
Of course, I know a lot of folks get their movies from the internet underground, and they've got us all beat for cost effectiveness... but that's something else.