Why wouldn't you download and play through the demo a few times before you choose to do so? You can assessment of how you like the actual game and can still pre-order. That's the idea of having a demo.
I thought you had to preorder it to get the game and it was only in a locked 'Demo Mode' until a certain date....'course I did drink a lot of beer and killed many brains cells last weekend so I could be wrong....
EDIT: ah, I see you're right, it's now available as a separate D/L...grabbing it now!

Left 4 Dead 2 demo now available to all PC users
Posted Nov 3rd 2009 7:00PM by John Callaham
For all of you cheap skates who don't want to spend money ahead of time for Left 4 Dead 2, your time to play the demo for Valve's upcoming zombie co-op shooter sequel has come. Now everyone with a Steam announce can download the demo for free.
Yes, you can check out the campaign and solo modes for the sequel as you run around the zombie-fied city of New Orleans while dealing with new boss zombies and using new melee weapons like our favorite the machete. If you like the demo you can still pre-order the full game for 10 percent off, plus get an exclusive Team Fortress 2 in-game hat.http://news.bigdownload.com/2009/11/03/left-4-dead-2-demo-now-available-to-all-pc-users/