Never check camera gear if you have a multi-element lens. One of the elements can get knocked out of whack and you would never know it until you looked at the photos...which would be slightly out of phase or focus. Not checking a computer should go without saying.
I used to check pretty much everything else, until coming back from an expedition in Mexico recently. Somehow, one of my reg's hoses got a puncture. It sure wasn't like that when I used it the day before I left. My LDS says they've seen that happen before in checked bags. I now carry on my reg.
I notice in Dominican Republic last week my mask seemed to be leaking, had to go to the backup...so I may start carrying my mask on.
Personally, I would carry everything if I could. But that is mostly because I almost always have to change planes and that is the biggest chance of a bag getting lost. Theft is really not much of an issue anymore and there is not much market for used SCUBA gear anyway.
A couple of tips, if you plan to carry-on a lot, try to get seated in the back of the plane so you board first, giving you more chance of finding open overhead bins. Remember, gate checking is probably worse than the regular checking process.
Also, airlines are now beginning to look at the weight of carry-on luggage. In Boston a couple of months ago, they had a scale at the gates. That bodes ill for the pack-mules who like to carry most everything.
Jeff