I have one and like it a lot. The competition would be the Korg X50, and I have that one too. The Yamaha was $520, the Korg $699, so its not quite the same price range.
The Yamaha has killer pianos, great El. Pianos, some decent realistic sounds. It doesnt have much in the ambient/synthy dept., its more of your bread and butter type sounds. I use it for the pianos and am very happy, they sound real.
The Korg is good for world, ambient, percussive, etc. It has some amazing combination patches. The pianos arent as good as the Yamaha.
Only rudimentary stuff like attack and filters. It is a preset machine. You can edit combis and splits and store them, but basically you use the sounds it has.
It is a cross between a PSR and a Motif. However, the sounds really are quite good. All I needed was the sounds so it worked great for me. If you need an editable synth, it wont be the one for you.
I just got the MM6 and the first piano patch is excellent. Very expressive and realistic. I have a 1960s Yamaha studio upright, and the sounds between the real piano and the MM6 are very, very similar.