I have heard that they should not be used until a baby is standing of its own accord (ie pulling themself up against furniture and standing for minutes at a time). They are also ok if a baby is already walking, or has just started totting around (provided there is supervision and it is not for prolonged periods).
However, they are not suitable before a baby is capable of standing as they can be damaging to their hips and lower back.
I have just received one as a hand me down- although DS has been standing for a long time and is now walking. He enjoys having something new to stimulate him and move around in a bit faster- although I only allow him to use it for short periods.