This picture is not the exact model from our gym, but it should give the general idea of what this machine is about, which is letting wimpy people do pull-ups and dips (where you lower yourself starting with your hands at your sides - the lower bars in this picture).
On this thing, the weights help you - they go up as you go down - so that you don't have to lift your whole weight. There are various places to grip it to work on different muscle groups. The machine at the rec center also has a way to remove the part you kneel on so that you can do regular pull-ups or dips too.
One consequence of this design is that you really must be able to pull yourself back up once you go down, because if you try to get off at the bottom, it's like jumping off a see-saw from the bottom - the weights will crash down and the thing you kneel on will fly up. So you want to err on the side of caution with this thing, but I had no problem doing about 20 pounds (it was too easy in fact), and I think I'll move it up from there pretty quickly.
What is really delightful about this thing, though, is just how smooth the action is. It just feels indescribably lovely to do it.