I built my first one with a 6610. Seemed fine, but I managed to get a 662 anyway, so I was curious to try it. Didn't really notice a difference, although it 'may' have smoothed the VCA out a bit, but it's really hard to be sure, and it's probably quite random anyway.
Put it this way, I got 2 X 662's, and i've not even bothered to fit the other 662 to my other x0xb0x yet.
I guess it's kinda nice to have at least 1 part in there though that is an original 'Roland' part?! But thats about the only advantage, if you can all it that.