I own both a Denon 2900 and an Oppo. Have them in the same system, hooked up to a JVC HD-ILA display (720p).
There are a few things about the Oppo that are comparable to the Denon. For instance, no layer change, speedy navigation. I can't live without those now.
My Oppo seems to play nicer with DVD-R, and is the only player I own that will play a dual layer DVD+R.
Picture wise, they are comparable (I have my Denon's output scaled to 720p by a Focus Enhancements CS-1 processor), but remember, the Oppo uses a Faroudja chip, so you may experience some lip sync problems (I never really have) and macroblocking (I do occasionally see this problem). Still, the picture is nothing to sneeze at, and for $200, an absolute steal.