Here's my opinion:
There is nothing inherently wrong with Behringer mixers. There certainly is an element of "you get what you pay for", but by and larger Behringers mixers do what the need to do.
Issues you may or may not see as a result of manufacturing inconsistencies include:
--a greater likelyhood of general failure (particularly in the extra cheap power supply that comes with the mixer)
--crosstalk in either the Alt bus or in the mains.
--knobs that are easier to break or damage.
A lot of these things are either covered in a warrantee or can be mitigated by handling your mixer with care.
Sonically I have found these mixers to perform as advertised. When I did a direct comparision (single mic soloed to headphones) between a VLZ-Pro and Behringer preamps I noticed that the VLZ-Pro had a warmer/fuller sound while the Behringer pre's were noticably brighter (almost tinny). Both were exceptionally quiet with little inherent noise. Both mixers mic signals' quality were degraded slightly when sent to the bus channels or the mains again with the Behringers there was a slightly more noticable difference.
All this being said I found little "wrong" with the Behringer mixers. There was a subtle audible difference in overall sound quality, but not so much as to label the Behringer as trash outright. In the end I did buy a Mackie board, but that was because I wanted the slightly better audio quality, and I was willing to pay more than double for it.
If you don't have the budget a behringer mixer will do fine, but it certainly won't be "the best"