I have three 8600 switches in my network. The only failure, like yours, was with an 8 port Gig module (the one with the replaceable GBICs) only in use about 1.5 months. I have NBD maint, luckily I had a workaround. The other two have been troublefree for over a year.
The only real gotcha that I have noticed with setting up and maintaining the system is that the physical ethernet port settings seem like they are buried in an inaccessable portion in the configuration tree, and you won't notice a config problem until you do some digging into the config. For example, I had a issue with a Gig fiber link to a 450 switch stack that had intermittant problems with. Finally figured out I had an autonegotiation mismatch between the 8600 and the 450. I have had multiple instances where these problems won't show up until a power fail or reboot on one end or the other, and the link won't come back up.
Otherwise, I have had good experiences with this product.