Conch's are great. I have both tiger conchs and fighting conchs. Got mine from LA but I don't see them on their site now. I gave up on urchins. They are cool looking but all mine ate was coralline algae, not to mention knocking stuff over all the time.
As a useful part of the clean up crew...no, not IME. The reason to keep a star is because you want to. I don't think any of them add anything of utility to the mix.