I have a 3" one in my 125 reef. I've had him over 3 yrs. Hasn't really grown that much, just his claws have gotten larger. He is safe with coral, and fish. No problems, as far as going after fish, etc. They are opportunistic though, so any fish, snail, or crab that gets too close and is too slow to move away will be eaten. I have very few nassarius snails, lots of shells though. LOL> My mandarin has a chunk out of his tail, I am almost certain it came from the lobster.
Mine has a cave that is his, and will defend it from even my large tangs. But, I wouldn't say that he is aggressive. (meaning trying to kill fish)
In my opinion though, they are not really good in a full reef with tons of rock, they hide all the time, except for when the lights are out. I see mine maybe once a week, if lucky. And only for a brief second, very shy.
So, if you really want one, I wouldn't put it in a tank with too many caves, etc. Other than that they are easy to take care of.