I have recently done a comparison between 818 and the pioneer 1522.
1. Spectacular upscaling (zero adjustment required and it provided a much better image of my satellite, netflix and DVD) - smooth, sharp, beautiful colors. So if image is your primary requirement, the Pioneer is it. It not even close on this front. Even though the Onkyo has a great upscaling chip, despite my best attempts, tuning the color to pleasing levels was just not feasible. The skintones are just off and horrible.
2. Looks - Looks much nicer than the Onkyo.
3. Price - If you get the costco deal, you have a $200 cheaper receiver compared to the Onkyo ($100 if you go with the onecall open box deal for $699)
4. Pioneer reliability is generally considered better (I have my 10 years old vsx-45tx still working just fine), although the Onkyo 818 has been pretty decent
5. 9 channel amps (not an issue for me)
6. Runs cooler - May be a factor if you are placing it in a tight space.
1. I think the Onkyo has a better sound for my preference. I know there are folks who prefer the digital amp sound of the pioneer, but I am not one of them. Onkyo has a more open sound, easier tuning with xt32. But for music, depending upon your speakers, the clean sound may be a bit grating. The high frequencies in the Pioneer seem more rounded.
2. Warranty - Pioneer has only a 1yr warranty, while Onkyo has a 2 year warranty
3. weight - Onkyo appears to be a more substantial amp since its heavier with more robust panels.
From my perspective, I am going to try to tune the sound on the pioneer since I cannot give up the image quality now that I have experienced it. The Onkyo is going back. The weird thing is that I seem to be getting used to the pioneer sound that it seems to be growing on me.
Hope this helps.