The laser unit is like a microwave. It produces the same effect whenever the button is pressed, no matter the machine. So the main difference is user operator. Be sure you're going to clinics with licensed (preferably certified) laser operators. A good operator will do a good job, irregardless of the machine. Most good operators tend to be found in more legitimate establishments like derm and plastic surgeon offices.
For laser hair removal to work two things are important. The have to use the laser most compatible with your skin type. The darker you are, the more chances you'll get burned due to the high melanin concentration in your skin that absorbs those lasers too well. White people have sparse melanocytes(melanin cells) so they rarely get burned with lasers. The ND yag works better with darker skin because the laser isn't absorbed as easy, therefore less burns. Unless you're a fair latino or lighter, ND Yag should be your laser of choice. If you're a real dark individual (midnight boyz, etc), laser may not be the best choice.
You'll need about 4-6 sessions for the laser to fully eliminate your hair growth. I had it done primarily on by beard due to me having an issue with razor bumps. Not that i've discovered some new pimple creams and beards are in, i'm low key regretting that decision.
Laser radiation in theory can and may result in cancer, but that's more an issue for the laser operator not you. Coming in once every 6 weeks for a treatment is no where near enough radiation exposure to be of concern.
As far as body parts and user demographics, biggest users are Arabs, Indians, and Gay men. Some women as well, but they tend to prefer higher class establishments. But as far as men, mostly gay men tend to come in for this procedure because they tend to hate hair. Brazilians are popular amongst more and more women though, as many hate the alternative (wax), etc
As far as clinics go, search for those affiliated with a plastic surgeon or Derm first. They're feeling the groupon burn too so they are desperate for business and will likely cut your a deal to get you in the door. A good strategy is once you see a groupon deal from a local plastic surgeon's office, call them direct and ask to pay them direct for the groupon deal. They'll almost always agree.
If you can't find a dr's office giving a deal, then look for clinics highly rated on yelp or google.
Lastly, for all this info, i expect some pics of the finished product....
Anyways, hope this helps!