Welcome to the weirdness. You picked a good nickname.
SHIII, and subsims in general, can be as hardcore or as simple as you want them to be. I play some flight sims, but I usually like to fly out the whole mission rather than leap right into the action, so I'm obviously pretty hardcore with my subsimming as well. SHIII is so pretty that I like to sail in and out of the harbors in real time, just watching the scenery. But you don't have to do that, nor do you have to take days to get on station. The time-compression is fairly high at 1024 x real time, and can be set even higher; or you can play the single missions, all of which start you right in the thick of it.
Take some time and look through the threads here and on the SHIII Mods board. There are plenty of options to choose from, most of which aim to make the sim even more realistic, but some which intentionally do the opposite.
As to whether it's worth spending the money: if you want to experience life in a U-boat it's the only real option these days. Go rent
Das Boot if you've never seen it, preferably the 5-hour "Uncut Version", but at the very least the 3 1/5-hour Director's Cut. If that turns you on, SHIII might be the thing for you. If not, well, there are lots of sims and games out there for all of us. SHIII does indeed have interaction with the crew. There is a full watch crew on the bridge, and they talk to you. The control room crew could use a few more men, but they also tell you what's going on, and you can read the guages over their shoulders if you like.
Multi-player is co-op only, no head-to-head. For that reason I've never played SHIII MP myself. Somebody else will have to describe that to you.
Good luck in your quest for your next favorite game.
