It's this freedom to take them anywhere that gives them their magic quality. A folding bike will give you entirely new traveling options. Well, weight is important, because you'll probably be lifting it in and out of the car, etc. Price is important too, as this will probably not be your primary bike. Folding bikes are affordable, and they keep their value.
Folded size is important also.
If you're just taking a couple of folders with you by car for a day or weekend trip, that's one thing. Almost any folder will fit the bill. But if you expect to commute by air, rail, or bus, then you'll need a really compact machine that folds quickly. Plus, pick a folder that's light, compact and quick to fold, and you'll always be one step ahead of the bike thieves, because your folder will always be inside with you. Usually one size will fit all family members.