OpenSolaris still has some Linux copying to do
Sun has made good on its promise to deliver OpenSolaris, the company's Unix-based answer to Linux, with a company-supported, commercial update arriving in mid-May. Although far from a complete product, the latest OpenSolaris is impressive and in the long run could prove a viable alternative to Linux.
Part of OpenSolaris' appeal is that it contains a subset of the source code for the Solaris Operating System, but with an open source license. Among the familiar Sun features are the enviable DTrace tuning and monitoring tool and the ever-impressive ZFS filesystem, neither of which are likely to make it to Linux due to licensing and personality conflicts.
On the other hand, larger Linux efforts like Debian (which is the basis of Ubuntu) have an impressive range of open source software packages which, so far, OpenSolaris can't match.
However, given that OpenSolaris can potentially expand its package support far more easily than Linux can start shipping a DTrace equivalent, OpenSolaris may prove a powerful competitor in the years to come.
As it stands we wouldn't recommend OpenSolaris to the casual user; there's enough gotchas and quirks to make running OpenSolaris a bit more of a headache than Linux (of course that largely depends on your hardware).