Sometimes.
If the game in question is from a series or developer I know produces quality titles, I may be more open to the idea preordering. Or, if the game has a good enough demo/beta preceding it's release, and I enjoyed myself enough playing said demo/beta, then again I'd be more willing to consider preordering.
However, I generally don't do it too early on nor do I preorder anything that isn't discounted in some way; either by lowered price or shipping with a host of extra "goodies".
Likewise, I will only consider preordering games whose development I've been keeping tabs on. Even so, I usually wait for the first few batches of reviews to show up before placing my preorder.
Take Xcom: Enemy Unknown, for example. I preordered it. However, I did so only yesterday.
I found the demo quite enjoyable, but that wasn't enough to convince me to spend fifty bucks.
After seeing the first batch of reviews though, and getting a sense of the passion Firaxis had towards the property during development, I was convinced enough to prepurchase.
[sub]'Course, getting Civilization V and other "bonuses" along with the preorder helped.[/sub]