Here is some info that I had found a while back on them. I have no personal testimony, but if you do a search for purple lobsters on this site I think this sums it up.
Lobsters are opportunistic feeders, that will eat everything and anything they can get ahold of. That includes inverts, fish, algae, etc. And they grow rather quickly.
If you do get one, be sure to keep it well fed
I know that the books say some lobsters have been successfully kept in tanks with fish, but unless you're very careful to keep it well fed, I wouldn't trust one. They are opportunistic feeders and any slow moving fish (like clownfish) or bottom swimming fish can be considered dinner eventually. It might not happen right away, but give him time to grow and you might have problems.
They also will dig out a home in the tank so make sure your rockwork is stable. They come out mainly at night so target feeding and keeping track of them can be inconvenient. If you do get it you may not see if for long periods of time because they do hide a mojority of the time.