I'm expanding my collection as people move to 4E. The second hand market (online and at the LFGS) is allowing me to pick up copies of books I've wanted for a while at a pretty good discount, as well as replacement copies for worn out or damaged books. It's a good time if you are planning to stay with 3e or are only dabbling with 4E.
Amen there. The amount I've picked up in the month since release is great. Most of which I would never have paid cover price for.
Good time if you're not moving on to 4e.