The 809 has an older video switching and enhancement circuit than the 818, which can support a video format that only exists to date on two monitors, both exceeding $20k in price. The 818 also integrates better support for mobile devices, something the 809 accomplishes poorly and awkwardly with an add-on BlueTooth dongle. If you have a laptop or desktop computer, that "improvement" is a fifth wheel. The 818 has a slightly better audio processor, which you'll never notice unless you have a truly excellent source, better cables, a good power filter, and better speakers than I'd expect most people to connect to this receiver. Both have an included mike, as does the 717, which is used during the initial setup to profile output equalization for room acoustics- which really makes a difference. Both the 809 and the 818 have 135 watts of power; the output formats of both support more speakers than your wife will tolerate- I believe the 818 can support two more than the 809, I'd have to look it up. Both do multizone support. In short, the difference is likely to be inconsiderable to most customers. The 717 has less expensive a/v processors and a 125 watt amp- but it's still a 7.2 multizone unit which will put a lot of older and more expensive gear to shame. The inverted Darlington output stages generally produce a remarkably sweet, almost tube-like sound, and any of these units will do well in small to medium rooms for home theater or even a bit of serious listening (in Pure Audio mode) with efficient speakers. I bought the 809, which is currently a very good value for the investment- and, having been in production longer (no longer being made, actually) has a stable operating system. The 818 and 717 are probably fine- but as someone who has worked as a factory tech, early adoption is anathema to me- eschew it unless you enjoy sending a unit back in for modification two or three times.
I looked carefully at the 818 after analyzing how I would use the AVR over the next four years. I'm into high end plasma panels but don't plan on moving to 3D at home nor 4k nor would I be connecting mobile devices directly to the unit for playback. Evaluating the surround codec support and power output, the 818 seemed like a waste of money. I agree future proofing is wise but the improvements are hardly worth an extra $250+. MHL would be cool but not needed at this point. The 809 produces excellent sonics so I recommend getting one especially if the price decreases a bit more.
717 doesn't have as good of a room correction capability, less power. 809 is very similar to 818 except 818 has Multi eq32 room correction software 9much better) and it has newest DTS Neo codec for bluray sound processing, power is the same. Also chipsets o 809 is better than 717. If you visit Onkyo website you will be able to see a complete differences between all 3 models. 809 was released in junw 2011 717 and 818 were released in june 2012.
They are all good receivers. Here is the web site that allows you to compare the differences. www.us.onkyo.com/prod_class.cfm?class=Receiver Once you open up the site click on see all receivers. There you will find each one. You will notice a small box to check on each model to compare them. After checking each model, click on compare at the top of the screen, and all models checked will come up on your screen. Then it's all a matter of what you want and how much you want to spend.
I don't really know I just bought this one because it does everything I want it to I can hook all the components I have to it and just run 1 hdmi cable to the tv regardless of what cables I need to hook it to receiver ( I have nes, wii, ps3, xbox360, satilite dish, blueray player all using diferent cables some regular RCA some hidef component some hdmi) it up converts everything to hdmi so just the one cable goes to tv and I use it as a preamp for my main front speakers and power them wirh a much more powerfull amp and use the receiver for the surrounds I also have it hooked to home network and can either listen to pandora and other Internet radios ( I only use pandora it's free others require subscription) or stream music from my computer over the network (only works with windows 7 or better) you can also set up speakers in other rooms but when you do this you loose 7.1 for5.1 and while your watching tv in the main room the only thing you can listen to in other rooms is thingshooked up by analog cables unless you run RCA jacks from the receiver to power amps in the other room
818 is the 2012 model and 809 is the 2011 model. Price on the 809 was considerably less. I paid about $450 for the 809 NIB on amazon on 12/31/2012 (one day only price). At that price I didn't really care about the other differences. The others answering this question seem to have a much better handle on the technical differences. Still love the 809 after 6 weeks.