I own the Cowon S9 32GB player. The Cowon J3 is merely a slightly upgraded version of it with a SD expansion slot.
If you are looking for an extremely easy to use, slick interface interface, then stop reading here.
Cowon S9 / J3 series basically have the best audio quality of all portable media players which is one of the main reasons I bought the S9. It supports many audio formats, especially FLAC which is the format I use. I recommend replacing the headphones it comes with 'cause like any default headphones, they all suck.
Video quality and colors are pretty good. The screen is a glossy 3.3" AMOLED which helps makes colors vibrant. I use it to play XviD coded files, but it also supports DivX and WMV which I never tried. The good thing about the S9/J3 is that it can playback video files that are higher than the resolution of the screen (480 x 272). It can playback my DVD rips which are 720 x 306 or 720 x 400 without any problems. So there is no need to re-encode to a smaller resolution. I don't have any 1080p material, but I think that's asking too much.
The interface is a bit basic and slightly quirky, but if audio and video quality is at the top of your list, then this fact should not deter you from buying one.