Summary: I bought this a few years ago and have just replaced it with a newer model with HDMI inputs. Overall the Onkyo is a good little receiver for the money but with a couple shortcomings. The lack of HDMI inputs is of course the biggest one if you want to use it for surround sound. You have to run the HDMI output to the television then run a digital output to the receiver, either coax or optical.
Summary: As all Onkyos, this is a good amp/receiver, and should give you good service until the"next" thing comes on the market. It does a lot. Two minor negatives though (or perhaps not so minor): it does not have HDMI input, and the speaker terminals in the back are so cramped, it will drive you bonkers, trying to connect all those wires. If you have a pianist in the famly, try and get him to do it for you.
Summary: I bought this on sale for $170.00 to replace a tx-sr502. The 502 was 5.1 and not xm radio compatible. Thought these were the major differences with the 504. I don't care about 7.1 since most MC is 5.1, and not a big XM radio fan. So as long as the 504 had all the other 502 features, the 504 would be fine and probably better. Not quite. The 502 would output any input to the B speakers, handy for outdoor speakers and extensions in other rooms.
Summary: Unfortunately, I had to return this Onkyo because it made my speakers sound just OK. I bought it based on reviews here and at Circuit City. It's highly rated, but my Monitor Audio Bronze B4 speakers sound great on my Marantz SR4001 (about $500) After hearing the speakers in the store on a Marantz SR4230 and then this TX-SR504, they sounded flat. The speakers are only $600, so I didn't think they'd be over the Onkyo's head.