I have an ARES SL-8. Externally it's almost (but not quite) as good as my Classic Army G36 (which I consider the best G36 on the market).
Internally the SL-8 is a standard V3 mechbox -no proprietary stuff. Quality is good. Had it out to the field for my first game last weekend and with the exception of one occurrence of misfeeding, it performed flawlessly.
So, if you were looking for an SL-8, I'd say grab and ARES one without hesitation. For any other gun... heed the advice in the thread.
"Someone in a Prius tried to race me at a stop sign the other day. I couldn't believe it. I had him for the first 100 feet or so but I can only walk so fast."