Funny you should ask the same question as me about 1 month earlier. Check this thread out: http://www.epic-ski.com/ubb/Forum1/HTML/001212.html
I've talked to everyone at the shop i work at too, and they said that I should invest in a set. I'm planning to do so once i get this month's paycheck. The two conditions I know of where you don't need a custom footbed is if you're 100% flatfooted even with your foot off the ground or if you have a 100% infalliable arch that doesn't even begin to collapse when you start edging.
The reason for the flatfootedness is, even if you get a custom cork...the cork's gonna be flat...a waste of $100-150 you're spending. If you have an infalliable arch, you don't need it either because that's what you're trying to maintain with a footbed...an arch that doesn't collapse.
This is how I understand it. The instant your foot is placed on the ground, all the weight of your body is directed downwards to your feet. With all the pressure on your feet, your arch collapses and your feet get "bigger". Think of a piece of playdough shaped exactly like your feet...when you put pressure on it from above, the area with the lowest structural integrity collapses first...that would be your arch...as a direct result, the footshaped playdough gains more surface area on the bottom. That I believe is what happens to a normal foot.
Beyond this, when you're in a boot, you require a rolling motion from side to side to get the ski on edge. Take off your socks and stand up and roll from a vertical position to the left or right and look at the arch of the outside foot. The arch deforms further right? When you perform that same motion with a boot without a footbed, the same thing happens more or less because the stock footbed has...well...no support. When you roll to one side or the other, most of the energy for turning is initially diverted to the arch deforming. THEN you get the bulk of your energy going into the ski for edging.
When you have a custom footbed made for your foot and inserted into your boot, the cork which is dense and rigid, holds up that arch under all conditions. So, during turn initiation, the arch cannot be deformed (being supported by the cork), thus, the energy is transfered into the ski more quickly. Atleast, that's the theory behind it.
From my understanding, thus, custom footbeds are not just for comfort, but for performance enhancement as well.
Another thing you should consider is future medical complications. I'm sure you're aware, but improper foot alignment and such are acutely related to migraines, headaches, and other annoying ailments. Proper footbeds prevent that.
So, it comes down to this...is it worth plunking down the $$ for? Seeing how long they last...yes. You're paying an average of $12.50/year if they last 10 years which is common.
Will it enhance your skiing performance? Probably...but to what extent, it's hard to say.
Which ones to get? Well, there's Superfeet which are cork, and use a non-weight-bearing process, DFP which uses dense foam and a computer, and a few other ones out there.
What molding method is better? It's hard to say. There are people who say the non-weight-bearing process is better because it's the natural conformation of the foot that's being molded. Others say that an imprinting method is better because your feet are "normal" when you're standing. Which way to go is up to you to decide...and also the stores to some extent...superfeet is available at most shops I'd think, especially bootfitters. The other ones may or may not be available.
If you can wait a month or a month and a half, i'd be able to tell you a difference since im' in the same predicament as you. If it's not possible to wait that long, I'd definitely say go for it!!
Hope this helps...if there's any inaccuracy in here, i'm sorry...and i'm sure someone on this forum will catch it :O