I've no beef with Denon, Yamaha or Pioneer in the $500 price range. They all have nice offerings.
I'd pass along another vote for the Onkyo TX-NR709 though. It's the least expensive receiver to offer full pre-outs. To me, that's a big deal since it means you can always add external amps whenever you want, or use self-powered speakers like the Focal professional or Emotiva professional monitors that I'm quite fond of. The 709 is also the least expensive receiver with Audyseey MultEQ XT - which offers substantially more correction of the bass frequencies vs. regular MultEQ or 2EQ or the YPAO or MCACC programs that you'll find in Yamaha and Pioneer, respectively.
There always seem to be some reports of problems with certain Onkyo receivers. And it's a consistent theme that they tend to run warm or hot and thus require good ventilation and free access to fresh air. It's been an ongoing thing, which is why the general talk on Onkyo always comes with some reservations. Their customer service does suck. So anyone who's been the unlucky buyer of a bad unit, I totally get why they might warn people away from Onkyo. It's a bit like how Toshiba still battles some bad feelings that come from, like, 20 years ago when their TVs always broke
But, on the other hand, Onkyo has been offering more features, or offering the newest features first, and they've been doing it at, very often, the lowest price points. So part of it is just that there are A LOT of Onkyo receivers being sold and thus, more units out there that could potentially have a problem. It's a bit like being Toyota and having a bunch of people who press the gas instead of the brake start making it seem like you have a problem with your cars.
Bottom line, I still recommend Onkyo receivers - especially the 709 right now. Just buy from an authorized dealer so that you have warranty protection. And if you DO happen to have a problem, get on it right away and stay on it until it gets fixed. The other brands aren't 100% immune to occassional problems either. A lot of the "Onkyo has problems" talk is just the "echo chamber" effect of message boards and forums: people who've never heard or bought an Onkyo, but they're just repeating the reports from ONE person who did have a problem. Forum people can be a panicky bunch

They're also binary - products are either a "10" or a "zero". No where in between
