I pre-ordered a game for the first time last year, relatively close to Christmas (actually, it was about 2 or 3 days before the release, but it still counts xP ).
I pre-ordered Persona Q for two reasons: 1. it's my current goal to build up my MegaTen collection (but that's been irregular and opportunistic at best), and 2. I assumed that it might take the route that P4G did with the PS Vita, and go up quite a bit in value.
Additionally, it had some nice pre-order bonuses. Even if a game isn't fantastic, getting a case, an artbook, a CD track, and some cards for $80-90, along with a cool box and the game itself, pretty much makes the purchase worth it in my book... but I'm a Persona fan, so obviously I'm biased. xP
Generally, I don't pre-order, nor do I pay full price for a game. It's a lot of money for a product that you may or may not enjoy. Recently, however, I've actually been MORE inclined to pre-order some games. I'd like to pre-order the new Danganronpa game (as much as I have my critiques about the series, I really admire what it tries to do, albeit imperfectly and what sometimes feels in a half-assed way), Persona 5, and (if I have a Wii when it releases) SMT x Fire Emblem. Zero Escape 3, when it hopefully gets announced, will also be a pre-order for me.
Again, for Persona games I at least have the excuse that I'm trying to collect games in the series. But, for me, these series have proven themselves to be good quite consistently; I like them, and even if they aren't perfect games, I can be pretty confident that I'll get my money's worth. Plus, as has been aforementioned, sometimes you get some pretty cool pre-order bonuses. I didn't pre-order Persona 4 Arena Ultimax not only because of money issues... but because those pre-order bonuses were piss poor. :/
This all is probably being written a bit randomly, but I hope it has some structure. xD I used to hardly ever give a thought to paying full-price or pre-ordering games. In general, I don't think it's a good idea, and I don't really need to say why; I'll hand you a list of the latest, largest pre-orders and a list of reviews of the games by independent reviewers, and it explains itself. xD But I've been finding some titles of interest lately. I don't complete a lot of games nowadays, but games like Danganronpa are something I actually do finish, so I don't mind paying retail for it (or, preferably, a used version).
Also, I will say that I'm more willing to buy higher-priced games in the summer, due to having a summer job, and consequently more pocket cash. xD
So, TL;DR - If I like a series, like the pre-order bonuses, or feel that the game will be worth a lot more in the future, I might pre-order, IF the money is there (a problem for college students xP ). Theoretically, I might feel more inclined to do so with something like import games, where sales figures are more important for finishing a series or getting a sequel.