Although it's not as in-depth as reviews by people/groups that have individually owned multiple folding bikes, Transalt.org has a great intro article to folding bikes, including links to several major manufacturers:
Wikipedia also has a nice write-up, and if you look closely you may notice that it's authored primarily by one of the forum members here. So, grain of salt, if you think the folks here are biased.
It was the wikipedia article that originally led me to bikeforums.
I'd recommend reading these two articles, and then start to set your personal priorities:
-Main use of bike.... commuting? Light/heavy touring? Off-roading? Plane-travel?
-Main reason for a folding bike.... small storage size? Security (bring it inside your apartment or work)? Multi-mode commute on public transit? Fit in a car trunk? Multi-size bike (many folders can fit multiple sizes of riders because of their telescoping seats/handlebars)?
-Tolerance for strange stares, comments, questions, "riding a wierd-looking bike".
-Wheel size, 26"/20"/16" (these seem to be the main sizes.) This will be the biggest area of trade-off... smaller sizes fold more compactly, but sacrifice ride comfort, unless suspension is added to the mix (shocks, sprung saddle, etc). 20" wheels with some sort of suspension seems to be a middle comromise for many.
-Size of folded bike
-Quickness of fold
-Price. If you're looking to purchase *new*, then many of the folks in here seems to think that $250-$300 is the minimum you can spend and get a folding bike of any quality... Not even everyone in here agrees on that, and each person has a different idea of what "acceptable" is based on their riding habits, expectations and income. If you're willing to buy used, you can shave some off the low end.
-Upgrade path/customization. How many parts of the bikes can be replaced with standard parts? Are you willing to accept a bike with lots of nonstandard/proprietary parts if it meets your other needs best? How available will the tube sizes be without having to order them over the internet? etc etc
-(I saw this in one of Wav's posts, and agree) Total Cost of Ownership.... based on *your* priorities. A less expensive bike, that can be upgraded later, or a custom-sized-and-kitted bike that is rather expensive but *exactly* matches your biking needs? Is resale value important to you? Include "time spent" in TCO.... if you plan on having to find odd parts, have to mail-order custom tire sizes, etc.
I hadn't heard of folding bikes before two months ago, and only have my first one on order right now....so take everything I say with a grain of salt. I feel qualified to post here because this is the same information-filtering process I had to go through.