Not many immediate negatives.. Unless you're using DXO? I believe DNG isn't compatible with DXO. Others complain of how much longer the import process takes because of the conversion. As far as differences between the two? Not a whole lot. RAW has the xmp side car files for the metadata, DNG has it embedded in the file it's self, not really a concern if you use Lightroom. DNG on average is 20% smaller than it's original RAW.

Me, I just use CR2.