I've been with Netflix since last May.
I was on 3 a month over the summer, switched to 2 a month once the new TV season started. This month I've had two times where I would send a movie back, they would get it the next day, but not ship my next DVD till the day after that. The first time it happened, my queue said it would ship that day, then when I checked in the afternoon, it said it was shipping the next day. The second time was this week. I sent a movie back on Monday, it got there Tuesday, and my queue said my next film would be shipping Wednesday. I sent a feedback reply about the first one.
But the pop-up info window says:
A movie in Shipping status means we have chosen that movie to be the next one shipped to you. Generally, we ship all movies within 24 hours of their entering Shipping status. Please note that due to inventory issues a movie may occasionally be in Shipping status for up to 2 days. If this occurs, we will automatically cancel the shipment and add the movie back to your Rental Queue.
Last year I had Season 1 of Battlestar Galactica in my queue even before it was released. The first disc was "very long wait" for maybe 6-8 weeks, so I left feedback about them needing to get more copies if people had to wait 2 months just to see something. I also threatened to cancel my subscription, IIRC, because it was ridiculous having to wait that long for something. Disc 1 changed to Long Wait soon after, and I got disc 1 very soon after that. Maybe they listened, maybe it was a coincidence.
I think inventory and when you get DVDs depends on your local distributor and whatever they have and when. I know there is a new Netflix dist center in my area, because it popped up late last year two buildings down from where I work (about 15 min from home).
Other than a few issues, and having to wait a long time for a few things (finally am getting Wedding Crashers this week, still waiting on Serenity), usually if I put a DVD in the mail on Friday or Saturday, it arrives there on Monday and I can get a new Tuesday release delivered on Tuesday. If I miss getting something in on Monday, that's when I have to wait extra long to get it. But I looked at my rental history, and I returned 10 DVDs in January, on a 2-at-a-time plan. $1.50 a rental. Not bad. But I like the selection as well. They have a lot of independent, foreign and documentaries that aren't as widely available. I wouldn't have enough to rent/watch with just the major releases if it weren't for the smaller, indie, foreign, documentaries and all the back catalog stuff they have.