It depends what you're cutting. If its something pretty thin, like laminate flooring, then a cheapo jiggly saw will be fine. Like Participant said, the blades are important, so get some Bosch, or re-braded equivalent blades.
If you're cutting something thicker then you'll want orbital action, but I wouldn't recommend buying a cheap one for anything too thick. You'll have to force the saw and it'll burn out in no time.
Second hand is worth a look, on ebay etc. People are always buying decent tools and selling them on after they get bored with DIY.