I'm sort of starting to wonder this myself. I'm looking to upgrade to an *upconverting* HDMI receiver, and my choices that I'm scoping out are a Denon AVR-4806CI and a Pioneer VSX-84TXSi.
I'm sort of leaning at the Denon - which hurts my wallet - because I just ordered an Epic 80 system from Axiom Audio, and have noticed that those are 4-ohm speakers. Pioneer's Web site says that the 84 can overload driving 4-ohm speakers (it apparently gets pissy at anything below 8 ohms), and I'm not really in the mood to buy a 1500-dollar receiver and then several separate 4-ohm amps to drive my shiny speakers. They both upconvert, and I'm hopefully going to go demo that feature on the 4806CI later today.
The Pioneer has the edge in terms of price (by...a lot), and from what I've heard it's fairly faithful to the 140 watt per channel rating. I have an Elite 74TXVi, which I absolutely love to death - the iPod connectivity is awesome, and it drives my current speakers (a nice, but inexpensive Onkyo 7.2) really well. I also really like MCACC as far as audio setup - the only thing it did weird was crank my subwoofer way way way down, but everything else was spot-on. I've heard some good and bad things about Audyssey, though. I also realy like the Pio remote, but I've never seen the Denon one.
So. Um. Owners of Pioneers and Denons, speak out! :)