I gave Solaris 10 more than a fair shake a few months ago (with an eye on its ZFS support) when I had a hard drive fail. I worked pretty hard at getting it to run and really didn't get very far. Note: I've been using Debian for almost 10 years now -- so I'm pretty biased.
From what I remember there was an astroturfed Sun-staff-only developer community, little information available online, slow as hell boot time, ZFS boot partition complications, and a broken KDE (the X server didn't work correctly; I have absolutely ZERO problems, even with 3D here in Linux).
And when I looked ahead to maintaining the system (the VAST BULK of where overhead is spent) I didn't see anything that looked as sane or easy as Debian. No incremental updates, just some arcane BSD-esque 'port' or .tar.gz package system (excusable for the rare unpackaged Perl module, but unacceptable for the whole damn system). I'm quite admittedly not very knowledgeable about BSD and Unix, but damn those systems seem like a bitch to maintain. And Nexenta simply wasn't there yet.
Solaris 10: pass.