Should I Trust Groupon Veneers for $400?

Unless the dentist is essentially do it for "cost" I would RUN from this deal.  

Groupon takes a portion of the deal.....so really the dentist is only getting $200 per veneer (approximate)

I can't imagine the quality of a $200 veneer

EXCEPTION: the dentist might be a new dentist and he's doing this promotion as a practice builder in hopes that you will be a patient for life. 

If I were you I would be VERY VERY Skeptical of this deal

I agree with the other docs -- that is a very cheap price for veneers.  There could be any number of reasons why these veneers would be so cheap.  If you're interested in pursuing a deal like this one or any other cosmetic dental procedure, there are some simple steps to follow to help ensure you get a good-looking and good-quality result.  (See link below to read these 8 simple steps)

A high quality lab will charge 250-350 to construct veneers.  That is before the dentist incurs all his costs of labor, rent, materials etc.  The only way they can even break even on these is to use the cheapest lab they can find.  Maybe there is a great lab out there doing awesome veneers for next to nothing but I haven't found one.  I can't imagine there is any guarantee at that price level either.  The true cost of a procedure such as veneers is not just the initial fee.  The value comes in quality and longevity.