I have a purple lobster, I never see him, he'll come out at feeding time, grab food and reverse back into his cave. A beautiful addition to the tank. Mine did eat my camelback shrimp though and if you get one, he'll more than likely do the same. Your fish, hermits, and snails should be fine though.
The first weekend I had mine, I looked for him and all I seen was his shell laying around in pieces, I was like oh s**t something ate my $35 dollar lobster this quickly. Then I found out they molt and there he was hiding safe and sound.
I recommend you get one, they are much more interesting then cb shrimp anyhow. So if your cb shrimp get ate, thats part of the food chain, right!
Gordon