I bought one of the Cambridge Audio BD650's a few months back. It seems pretty good - decent format support, build, reasonable price, better looking than most (I hate the shiny plastic on most players and all the DTS/DVD/Blu-ray/HDMI/HVCHD... logos). I think it's a bit slow at loading discs, but I think that about every blu-ray player I've ever used. I can't really say if it's the best sounding player for music because it's on a surround system that doesn't do music at all well.
Standalone CD players are still better than most multi-format players at playing music from CD for the same price. There can be issues with optimising the audio performance with the different sampling rates if it's not done well. There are also extra outputs needed on the multi-channel players and the copy protection and license fees for some of the formats and connections.
For me the biggest thing is the loading times. I like to drop a CD in the tray and listen without the clunking noises that DVD and Blu-ray players seem to make while they spend 30+ seconds loading. I know there are some very good players that don't do this as badly, and that it will get better in time, but right now they stop me using such devices as primary music sources. I've not messed around with music only multi format players as I didn't have any SACDs before I got a DVD player that did both, but I do have the odd music DVD and the Neil Young archives on Blu-Ray