We have used them for a year +.
They are convenient and have a very nice variety to select from.
What we like best is not having to worry about returning rentals to Blockbuster/Hollywood Video on a certain day. (Not to mention a few times at both places they claimed we never returned items and they had to physically locate the item on their shelves. So they are not infallible themselves.)
And never failed -- we never remembered until bedtime that rentals were due that night. Then poor DH is the running out at 9 - 10 p.m. at night to return them.
The variety we have rented from Netflix even includes IMAX films.
There are a few downsides. We have new DVD players. We have learned to check to see how dirty the DVD's are before we even insert them into the player.

They are seldom clean and skip and stop. We have had movies we haven't even been able to finish watching. One weekend we had 2 out 3. So needless to say we were not happy with them then.
We return the DVD's to MN. Even if I mail all three at one time -- physically handing them to the postal clerk -- they seem to show up at Netflix over a three day time span.
A few times the rentals have taken a long time to reach them, and I have even reported them as lost. A few days later they show having them.
I use the postal service a lot mailing pins. I have not found the postal service to be unreliable. It's really a rarity.
So we have had a few issues. But nothing of importance. Netflix isn't perfect, but it is a good option overall.