Disclosure: I have had very little hands-on experience with it, and I promptly put Ubuntu back on my laptop due to no touchpad driver.
Opensolaris is looking like the go-to operating system for server environments. Reading some ZFS usage docs, it's the best filesystem available for many reasons: Snapshots, transactions, and the combination of disk manager and filesystem are very appealing.
Of course, Xen is fully supported on Osol, so virtualization is as much of a breeze on Osol as on any other *nix system.
Crossbow, coming out in SXCE build 105, sounds completely AWESOME. Read about it here: http://www.cuddletech.com/blog/ (scroll down a bit). It's basically full network virtualization, which is very very cool. You can literally create a virtual network infrastructure on one machine.
Sun's websites and clarifications on some things could use work, but their user manuals are incredibly useful. The package naming scheme in IPS will be cleaned up in time. I have some exploring to do w.r.t. Sun Studio, Netbeans, and other development related topics, but I'm really liking the prospects of Opensolaris. It will likely be the host of my server, which I just massively upgraded (1.3 TB HD space, 6GB RAM, Antec Nine Hundred case). I'll probably still run Ubuntu Server as a VM and play. Along with IPCop, CentOS, and Windows Server 2008.