Absolutely first rate.
We've currently been able to provide 100% uptime of our file&print, dns/dhcp, e-mail, internet access and web services using clustering. We are able to being servers down by migrating services to other servers while the work takes place.
Even the business love it! The only thing I would say is that if you are looking at clustering critical information, you may consider purchasing the additional 3+ node clusting option, as NW6 only supports 2 out of the box and these can *sometimes* get it wrong i.e. one box drops, and the other thinks it's the one with a problem so it take a poison pill and drops also.
Steve