Anybody who thinks a PS3 is as good as a dedicated blu ray player for playing blu ray discs is clearly a tool.
2/3 years ago it was a valid opinion - when you couldn't get a BR player for <£200. The PS3 was pretty much guaranteed to be constantly and easily updated for format changes, etc (3D for instance). These days you don't really need to spend much on BR player. The difference between a good one and a cheap one is very small. Very different to DVD. It's pretty much a digital transport - most of the signal processing is done by your TV and/or amp. As long as it can output/decode all the formats, and ideally is networkable for updates and live content, you should be fine. No point buying a separate player if you have a PS3, IMHO, unless it's a loud fan one. And if you're going to play games, why buy 2 devices?
I use a PS3 as my only player at the mo. I also have a HD-DVD player that I bought when the format died. Got the player for £60 and around 50 hi def films for 3-5 quid each. About 2/3 still in shrink-wrap admittedly.
I suspect if you put a PS3 and a new reasonably pricey BR player side by side on the same audio/visual hardware, you'd be extremely hard pushed to tell the difference. But not having actually tried it, couldn't say for sure. The quality of your TV/monitor makes much more of a difference. And of course whether you've calibrated it (or had it done professionally).