I live in Marshalltown, and my Netflix movies are sent from Des Moines. For me, I send a movie back one day, it arrives at the shipping center and a new one is sent the next day, and on day three I get my new movie. But if you are closer to Des Moines, it could theoretically be faster
I've only had my HD player for a month, and I have noticed a 'short wait' listed for the HD movies - mainly new releases. But that is probably due to the fact that we live in Iowa, and HD movies aren't in high demand! When more people want to rent the movies, hopefully they will have more to send out.
I've never had any real problems - I've only had two movies arrive scratched, and Netflix sends a replacement pretty quickly.
I've never tried the Blockbuster service, as we don't have a Blockbuster in M-town. I admit, it would be pretty convenient to be able to return a movie and get another immediately!
Hope that helps a little!