Mechanically, it's a very stressful range of motion for your rotator cuffs. That said, I know some big guys who swear by them, and all say start low and slow. No jerking the weight, use the ROM, and low weights with slow progression.
As for working the traps, a closer grip will put more emphasis on the traps, while a wider grip hits the lateral delts more.
If you want traps, I can't recco good ol' BB shrugs enough. I use them, and know many, many others with big traps who use them. Time under tension is one pricipal I use, along with heavy weight on the bar. Keeping the shrug up tight for 5-10 seconds each rep will really beat the snot out of them..and the heavy weight will shock them into submisson for a few days. And by heavy, I mean enough weight to keep your max reps under 3 solid reps.
Mt Snow, VT - May 5, 2012 - Completed!
...i remain, he who remains to be...