I have the Boots Smooth Skin. It got really great reviews, but Rio did not, Im afraid. After 6 treatments, I have quite a few "bald patches" and my body hair is v dark and course BUT it takes AGES per treatment - doing just the bottom half of each leg takes just over an hour (for both legs)
Its def worth it though - my legs havent been seen in public for years, but that will change by next summer
You are supposed to leave 7 days between treatments, and not use it during your period. You shave just before treating (hair can be max of 1mm length), and can shave as often as you need to in between.
The actual zapping is painless - literally a half-second of tingling pins and needles feeling (and Im a big wimp!) But the bit you put against your skin does get hot so I need to switch it off and let it cool for 10-15 mins (when i go give myself a facial or manicure/pedicure). This may not be the same for everyone - in fact, of the dozens of reviews Ive read, none have mentioned this.
I would like to do underarms, but you cant use deoderant for 24 hours after and I cant find a 24 hour period in my week when I dont need to be in public somewhere!
As for bikini, stay away from..um..internal bits, but you could do the bit closest to your legs / top of bikini.
You cannot use this on your face. I would imagine it is because of the extremely bright red light emitted from it damaging your eyesight and the sensitivity of the skin on your face.