Most of my images are Canon RAWs (CR2s) which I convert to DNG to avoid the need for XMP sidecars. I have the DNG converter embed full size JPEG previews (produces 3504x2336 previews from my 1DMkII's CR2s). The full size previews are good quality but too big for fast browsing in most software. I tried embedding medium size JPEG previews in my DNGs (1024x683 from my CR2s) - they allow fast browsing but are too small and of such low quality that I cannot use them to evaluate my images.
My CR2s contain an excellent quality embedded JPEG preview (1536x1024) right out of the camera. That size preview big enough and clear enough to allow me to rate, label and select images and browsing is very fast. I wish I could embed that excellent Canon preview in my DNGs.
In the alternative I wish Adobe would make the JPEG preview options in the DNG converter more flexible. It would be nice if we had a list of size and quality options so we could embed previews of the size and quality that we need. A high quality 1536x1024 option would be most welcome.