I have the Onkyo 898 and like it a lot, with two exceptions:
a) The Dolby digital decoder suffers from the "Pearl Harbor" bug, which means repeated audio dropouts on a few discs (Pearl Harbor was the first) that have been encoded with a different Dolby flag format which Onkyo says is out of spec and Dolby says isn't. The only work-arounds are to play the DTS track (if available) or use the DD decoder in your DVD deck.
b) They've had quality control problems with the DSP board that started with a bad batch of chips. They say it is behind them now, but I had to put mine in for warantee service twice.
The two main reasons I chose the Onkyo over the Denon were: #1 it has s-video/composite converters in each video channel, which was very useful at the time I got the receiver #2 it has adjustable audio delay, which is very helpful for keeping the sound in sync on my plasma, which has a 2-field video delay for de-interlacing.
Search in the Audio forum and you'll find lots of discussion of other pros & cons. Some people don't like the way the Onkyo does bass management. Some people think Denon has a nicer sound.