I currently have a Denon 3313 (though I'm probably going to get a 4520). The 3313 is a great AVR and sounds fantastic. I haven't tried the 818 in my setup but I actually replaced an Onkyo 876 with the Denon 3313.
In comparing the Denon 3313 to the Onkyo 818 I see the following advantages/disadvantages:
- Built in airplay
- No 24fps bug
- Overall very polished operation (very quick HDMI switching and syncing when the resolution is changed)
- MultiEQ XT versus XT32 for 818
- May have slightly less power than the 818 as the 818 weighs well over 10 pounds more - on the other hand tests of last year's Denon 3312 and Onkyo's 1009 indicate that any differences would be small - e.g. with 7 channels driven I believe hometheater.com reported 86wpc for the Onkyo and 80wpc for the Denon - an irrelevant difference that will yield no measurable difference in output (you would need 160 watts to get a 3DB increase).
The Onkyo does look like a better value on paper but it features many redundant features (e.g. THX Loudness Plus and Dolby Volume accomplish the same thing as Audyssey Dynamic EQ/Volume) that I know I wouldn't use. For me it was easy as the 818's 24fps bug was a showstopper. On the other hand, XT32 is great but in the 818 it doesn't include SubEQ HT like it does on the 1010/3010/5010 so I didn't find it as compelling - certainly not enough to dismiss the 24fps bug.
When you think about it, the 24fps bug will be visible but the the benefits of XT32 you would have to be in a position to do an A/B comparison so to me it's easy to see which advantage/disadvantage is more important.
Anyway, that's my $.02