I have one and i recommend them. I also have a steel cable that it locks into to make it that extra bit securer.
High-Tensile Steel Cable
The noise is plenty loud enough to draw attention and also warns you (as already mentioned) to remove it before riding off. The actual lock with out the siren is very good and solid.
I've had mine over a year now and it is still working on the same batteries.
The reason they go off on their own is due to the fact that they arm themselves by reflecting a beam of light. If that light is broken (by the disc) then they are armed. After a while the reflector gets dirty so it does not reflect the light beam and in turn, arms itself. If people just maintained it by using a cotton bud and some petrol all works very well.
[ This message was edited by: Sparti on 2006-06-16 06:35 ]