I have to admit I've been somewhat skeptical about the actual uselfulness of OpenSolaris. Yes, Solaris is one of the most advanced operating system in the market with quite a huge installation base, but yet it was hard for me to see where OpenSolaris could acually fit in the open source landscape.
Some days ago I found project nexenta and I have to admit I'm impressed. Long story short, Nexenta is a comunity driven attempt to build around the open solaris kernel a nice Debian based environment... and it actually works pretty well.
Open Solaris is a very interesting product by itself, but until today I could not see how - in a world where application portability is by far more important than, say, a strong kernel - one could do without the entire Linux ecosystem.
You can maybe recompile any Linux application under OSX, but it makes no sense to do so: the same goes for Solaris.
Back to Nexenta, I have to admit that ZFS alone is worth the price of admission, allowing for transactional upgrades in a well-known Debian environment.
Next time you build any Debian (or Ubuntu, that is) based server, you should really consider Nexenta: performances are, according to my quick benchmarks, impressive, and you still are in the familiar and cost effective apt-get world.
As a side note, nexenta's commercial project (Nexenta Storage Appliance) seems very interesting too, able to run on stock hardware with all the power of ZFS and all the usual administration panels.