We've got 4 * 6509 with sup 720 10gig on the assumption we would move to VSS once it had been out for a while and there was better functionality.
As part of the purchase we also had modular IOS but we've had nothing but problems with it and with Cisco saying that currently they haven't released any patches yet and they're finding a greater number of bugs in modular rather than non-modular IOS my question is, is it worth sticking with Modular or drop back to Non-Modular until the modular IOS is more stable?
If your 6509's are not in a redundany topology, and cannot have any downtime - the mod IOS is probably the way forward. If you have redundandy sup modules running sso mode in each 6509 - the you can force the standby to be the master, reload with whatver you need to do, then perform the same on the redundant/primary sup.
The mod IOS is a "feature" but not 100% yet...In a production enviroment I always prefer NOT to be a BETA tester for Cisco.
I haven't had a very good experience with modular IOS either. It is still relatively new and still have a lot of bugs to get worked out. If you are having problems I would roll back to the traditional IOS and give it more time before you put modular IOS in a production environment.