You can cancel before they have the disks back, you just have to get them back to them within 7 days of canceling. Read it.
6. How do I cancel or change my membership?
Canceling or changing your membership is quick and easy. Simply log in and select the Cancel or Change My Membership link in the Account Maintenance section of Your Account. Select the appropriate option from the list shown on the page and follow the instructions to make the changes. If you decide to cancel, remember that cancellation is effective immediately, and all discs must be returned to NetFlix within 7 days. For more information about this policy, visit the Policy section of the Customer Service Center.
As far as in stock items, that's their biggest flaw. I have a queue of 230 or so, and of the first 30, all but 4 are out of stock (but these are anime titles, so they probably don't have many copies to begin with). If you want a particular movie ASAP, you're probably out of luck with Netflix, but if you're just trying to get movies to watch, it's pretty good.