Just picked up a 640 yesterday and so far I love it. It starts up in just a few seconds (maybe 5) and seems to be faster to respond when recording manually. It is very quiet, (even quieter than my 633) and I like the new quick timer entry system which eliminates the possibility of messing up with am/pm. (I use satellite, and BTW auto-record doesn't work for my Starchoice 421 box either.) I haven't had it long enough yet, but my first impression is that SP recording quality may be a bit better too. And hurray, no EPG!
What I don't like as a 633 owner (I'll be sending it back soon as I accepted Pioneer's offer to replace it with a 640,) is the 640's remote for reasons already mentioned: silly inconvenient sliding door hiding buttons, missing buttons like Timer Record and Open/Close Tray, etc, and another oddity I just discovered:
The Channel Skip buttons don't work on finalized DVD's!?
It's not that the buttons don't work, because they double as left/right navigation and they work fine for that, plus I can channel skip anything that isn't finalized.
Interestingly, if I use the 633's remote I can channel skip fine whether finalized or not. This means the two remotes must send a different code for channel skip. (BTW: I'm using DVD-R)
Also BTW, all the 633 remote's buttons seem to work on the 640, including open/close, timer record etc that are missing on the 640 remote. I'm going to see if I can keep the 633 remote when I exchange units, and probably buy another one for the 2nd 640 because I'm accustomed to using channel skip.