Personally I'm not a big fan of pre-ordering - especially when all you've seen of a product is trailer videos and carefully crafted dev hype.
It's 'buyer beware' and rightfully so. There are, to be honest, quality issues with many games coming out these days, and you can hardly trust marketing to reveal that ahead of time!
But on occassion I have pre-ordered. Kerbal Space Program for one, because I played the demo and loved it. Gal Civ 3 because I love me a good space 4-x and I'd seen plenty of youtube let's plays and it looked fun. Guild Wars 2 because the beta weekend blew me away.
For me that's the secret: if you have seen enough of a game to actually judge if the gameplay seems fun RIGHT NOW, go ahead. But don't buy on reputation and screenshots, and don't buy a banal product in alpha hoping it's going to be awesome later on.