Got my 24" Aluminum iMac in early September, never experienced any lock-ups like I see described here. Upgraded to Leopard last night, no problems at all (just did a regular upgrade, no archive, etc.).
As someone else mentioned, at first it indicated it would take 1 hour and 37 minutes to install, but in the end it only took about 40 minutes.
I guess the only thing that seems a bit different is that my external hard drive seems to be waking up on its own for no apparent reason. I didn't install/configure Time Machine, so that's not it, but it never used to spin up unless I tried to access files on it, but last night (and this morning) it did.