I have been using Designs for Vision 3.5 magnification. Absolutely love 'em. I don't find it a problem at all to look over the top of the loupes for normal vision.
I don't buy into the "you're reducing your field of vision" stuff all that much. I can see the tooth I'm working on great as well as the proximal teeth. I am usually not that worried about what is going on in the next quad over at that time. I DO recommend that when you give shots you don't look through your loupes (at least not until the needle is about to penetrate) as classmates have been known to nick a lip on the way in if you're staring at the injection site the whole time.
If you just want to give them a try or have a backup set for different magnification, Rose Micro
http://www.rosemicrosolutions.com/ has some very competitive prices. A classmate bought some higher powered ones for endo from them and says they've been worth the money. They also have a pretty cheap light for about $120 if you want to try that out as well.