I am using for overheads on drums, acoustic stringed instruments (guitar, mandolin, etc.) and hope to try on piano soon.
The main difference with the c1000 is the high end. The c1000 seems less "open" or more "pinched" and grating on the high end. Also, the pickup pattern is much wider with the 603 (I could hear more of the room right off!) so you have to be careful about that, but that can be an advantage when you want to use something close to an omni but still slightly more directional.. it's a cool mic no doubt...
Someday soon I will try to do a head to head shootout between the two. Where could I post some samples?
-Doug