I've been using a Mionix Castor for a couple months. I can't say I love the ergonomics. I wish it were bigger (I had the Naos 7000 before it, but the finger shelves didn't feel the way I thought they would).
It has small finger grooves for the ring and pinky fingers, but it's too small to hold it in a way that the user's fingers actually fit into them (it's possible, but I always end up switching my grip because it doesn't feel natural). This just results in an uncomfortable bump on the side where it should just be round.
The sensor is really nice, and it is rather attractive. Scroll wheel is decent.