QuoteCR2 out of the cam (with and without ML): Lossless compressed
DNG/MLV/RAW out of ML (produced by some ML features, not all of them): Uncompressed.
This thread is about enabling ML to produce lossless compressed output, too.
And compression means less bandwidth required writing to card.
QuoteFiles are 15-20% smaller and image previews load faster in lightroom. The disadvantage is some metadata is discarded and dng files don't play well with some non-adobe software. There's also an argument that dng files are more future proof, but since old dng files are different to new dng files, I don't buy that argument; I'm sure I'll still be able to access/convert cr2 files in 20 years if it becomes and issue.
Basically if you never intend to move outside of Adobe's ecosystem then it's a nice choice for saving some hard drive space. With modern hardware the speed advantage will be minimal for most people, and there's the risk if you move to capture one or dxo (say) in the future, you'll have issues with camera profiles and lens correction not working accurately (despite the format being semi-supported).
Well, i still don't get it...