I posted this information on another thread in response to someone looking buy their first folding bike. I'm adding it here since it's general (and I hope useful) advice:
The difficult thing in making folding bike recommendations is that there are so many different types of folders out there, with each one engineered to solve a specific set of problems. For example, here are some factors to think about:
(1) Rider height and weight (bikes do have limits)
(2) Compactness of fold (this is critical if you're planning to use the bike for multi-modal commuting; e.g., taking it every day onto a packed rush-hour train or bus that doesn't allow full-size bikes)
(3) Ease and speed of folding (important if you're going to be folding it multiple times daily; if the fold is tedious and time-consuming, you'll resent it pretty quickly)
(4) Gearing (If you're riding in perfectly flat conditions for short distances, a single-speed bike can be more than adequate; if you've got hills, then multiple gears become important. The important specification is not so much number of gears but your *gear range*--30"-90" is decent for regular commuting in varied terrain.)
(5) Riding comfort
(6) Riding performance/speed (important if you've got a lot of distance to cover; say, 7 miles or greater at a stretch).
(7) Bike weight (If you live on the 5th floor of a walk-up, you don't want to be hauling a 30-pound monster upstairs all the time.)
(8) Quality of construction (Is it made well and with decent materials?)
(9) Customizability (Does it use mostly standard bike components that can be replaced and upgraded over time time at your local bike shop or does it use lots of proprietary parts and require servicing by specially trained mechanics?)
(10) Cargo capacity (Do you plan on using your bike to haul groceries, shopping purchases, children?)
(11) Cost (does it fit in your budget?)
As you can guess, there is NO one bicycle that performs equally well in all these areas. For example, a Brompton is a fantastic bike for its quick and compact fold, perfect for that crowded train commute scenario. On the other hand, it's expensive with lots of proprietary parts and not a bike you'd want to ride long distances on. On the other hand, a Xootr Swift is perfect for long-distance commuting (fast!), is highly customizable, and costs much less than a Brompton BUT the folded size is more than twice that of a Brompton.
Probably the best advice I received when starting down the (folding) bicycle path two years ago after years of not riding was, "Don't spend a lot of money on your first bicycle." What you really want is a bike that's just "good enough" so that it's not a total disaster, and that you can ride over several months to gain greater insight into your bicycling needs. Then you can make a more informed purchase down the road when you're ready.
For example, when I started riding folders, I felt that a compact fold was the most important factor and I didn't want to spend a lot of money, so I got a Downtube Mini. Over time, I discovered that compactness wasn't so critical for me but road performance was, so now I have a Xootr Swift.