Pharm, there's one thing your questions seem not to take into account, so I'll deal with that first. That is section 922r compliance. An imported non-sporting shotgun can only have 10 or fewer imported parts to be legal. In the case of the S12, the number is four or five (depending on how you interpret "muzzle device," since the S12 has a thread protector) In the course of a conversion like mine you end up taking care of that without a problem since the fire control group (3), pistol grip (1) and stock (1) will put you into legality. It's more of a concern with the bolt on "conversions" like others are talking about. If I recall correctly, a protruding pistol grip and detachable box magazine are what launches the S12 into "non sporting" territory. If you just swap out a different traditional shotgun stock, you should be ok.
Now, for your questions:
1) Am I happy with it? Yes, but I did this more for fun than for anything else. My most common failure with the shotgun are shells jamming against the upper part of the chamber upon cycling. I haven't really spent the time to deal with this, but it just isn't going to be as reliable as a Rem 1100 or Mossberg 9xx. As far as patterns, I've shot skeet with it but I haven't patterned it. It works ok for skeet; consistent cheek weld is the hardest part.
2) After conversion it uses a standard single hook trigger group. That trigger group is modified slightly- there's a manual bolt hold open lever on the right side, so the axle portion of the hammer (and maybe trigger, I forget) needs to be ground down slightly. Tromix sells a G2 trigger group that has had that done already. A worthy purchase for someone doing a conversion, since it's 3 compliance parts.
3) Most effective add ons: I hear folks talking about the Gunfixer gas plug as being the best thing since sliced bread. I wouldn't know, haven't used one- it's on my "to do" list, but I have been sidetracked by AR15 projects lately.