In moderation, I think they're fine. I tan about once a week, just to maintain some color. It works pretty well for me. I prefer the stand up units. They seem to give a more even and consistant tan.
I wouldn't use any form of sunblock or else the tanning will be useless. As long as you aren't prone to burning, start off with 8 minutes and work your way up from there. If you do things right, you should be fine.