I keep mine @ 76-77, he seems fine. Had him like that since November 02'.
They're pretty easy to keep so long as your tank is set up right.
Mine seems to like a little bit higher salinity.
Eats squid like a champ. He shed his stinger and I found it one day moving rocks in the tank, put it on the coffee table, and roomate threw it out. It was pretty cool. He's got another one now. Actually it is a she. I just always say he for some reason.
As far as how active they are. Not always active, but they rest for periods of 20 minutes plus sometimes and swim all night long after the lights go out. I've been thinking of adding some moonlights.