I tend to take preorders with a huge grain of salt, personally.
It's true that developers are growing desperate and are now considering preorder DLC to be some sort of Sine Qua Non clause for the release of a new title. "New game's coming out in a financial quarter or two? Damn, better iron out those Day One DLC plans, then!"
In an idea situation, preordering works out. Game Such-and-Such pops up with an announcement trailer, you follow a couple press releases or developer interviews, things look increasingly solid and polished as the months roll on, and then you see or hear something that seals the deal. You preorder, get your game a month or so later, and everything's peachy.
Now, we're looking at fiascos like Aliens: Colonial Marines. I know it's not a perfect solution, but my personal experience has driven me to be really careful with what's released to the public before a given game comes out. If there's too much hype, I start to smell desperation on the winds. I can almost see the number-crunchers begging the Marketing guys to find a way to bring in, oh, an extra twenty million purchases so they can break even on a stinker that's about to be released to the unsuspecting public.
Considering, I never pre-order the moment that option shows up. If I'm excited, I'll stick in the rinks for a few extra months and read up on everything I can. I usually pre-order near the tail end of that part of the release cycle, sometimes a day or two before the actual release. Taking my time and honestly researching really sudden gamer crushes, I saved myself from a couple awful flash-in-the-pan scenarios. I *almost* pre-ordered Colonial Marines, but the PR-based pressure was so intense I started getting suspicious.
What I really find difficult is less games that honestly don't live up to the hype, but cases where you're stuck with games that have diminishing potential over time. Spore was amazing at the time of its initial GDC demo, it was alright by the time of its release, and now I just wonder why I even have that particular disc.