Like Will, I owned a 905 before upgrading to the Arcam AVR600. I was happy with my 905 for movies, but not with stereo sound, music playback being the most important to me.
My 905 did run very hot and used to cut out over prolonged playback, however I only had about 3" of clearance above it despite it being in an open hifi rack. When I increased it to 7" I had no further problems. I must admit this does seem a bit excessive though and not everyone has that amount of space to play with. I sold my 905 for about £500 a few months ago (it only cost me £800 new from Richer Sounds). It was a more recent exmaple with one of the later firmwares installed which rectified some of the early problems with the amp. £600 seems a bit steep for a 905, if you do get a secondhand one, make sure it's got a later firmware installed.
I sold because I wanted better stereo sound, however before selling I bought a CA DacMagic and had it modded with Burson OpAmps. Plus I replaced my QED Silver Anniversay bi-amp wire to bi-amp runs of QED Revelation. Both of these additions made much improvement to the Onkyo's stereo sound, giving it a deeper midrange where before it sounded too lean and harsh.
If you don't want 5.1 sound, I'd agree with others, get your self a good stereo amp, something like the Roksan Kandy K2 or Naim Nait XS. But if you think you might go the whole 5.1 route at some stage, I don't think that the Onkyo on it's own will give you very good stereo sound. Having said that the AVR600 is a hell of a lot more dosh and isn't without it's problems at the moment. Another consideration is the
Yamaha DSP-Z7 (
£1650), supposedly excellent with movies and quite adept with stereo. Also a 2009 secondhand Pioneer SC-LX81 has just sold for £950 on the classifieds of another well known Forum of the AV variety, keep an eye out there.