Yes, they work... as long as the gunk is not too caked/baked on.
I use mine mostly to clean mouthpieces though, toss it in on a weekly basis keeps them nice and clean.
In general, I don't use them for slides/valves for a couple reasons....
1. the one I bought is too small for the larger slides.
2. the metal to metal contact in the bath can make some scratches on 'soft' metals .... hmmm, brass is soft....
3. if you use the plastic basket insert you get about 30% of the cleaning power and then you are really doing a lot of nothing.
4. since you can't throw the horn in there...what is the sense of having uber clean slides and a dirtier horn?
I do throw valves in there occasionally for horns I am donating or using as project horns or just messing with.