SmartOS is something many people overlook. It's an awesome OS. ZFS is IMO enough reason to try it out (no, zfs for linux is not ready). But then zones make it even more awesome (there was a discussion about zones here on HN a few days ago).

This is usually how it is around sysadmins:

Those who know Solaris very well swear by it and implement some awesome very stable running high performance and secure machine setups. Those who don't, hate it because it's not Linux.

It takes a while to become familar with Solaris from a Linux background. But the more you read about it, the more you realize how well designed it actually is.