Initially, due to popularity, I though Linux was the most convenient learning path to Unix™. In face of the many different brands of Linux I soon realized the opposite. I think it uses an ad-hoc design and its flavors are not consistent among each other, wildly varying in terms of vision, goals, features, integration, stability, support and life-cycle. I can help being somewhat straight but Linux environments are generally a mess or at least a poorly manageable mixture.
Solaris on the other hand uses a careful design and doesn't suffer from any of these problems. Thus, it provides a much more solid foundation one can rely on and build up a much more robust IT back-end infrastructure for the enterprise.
I also recognize FreeBSD as an excellent platform for the open source market.
AiX™ may be great if operated by its maker, not by SysAdmins, so all bets are off.
In my opinion, the main drawback of Solaris is the lack of applications.
That's odd as any seasoned developer can recognize its strong foundation and tools.
A fairly nice desktop, GNOME, has been provided in Solaris 11, phasing out legacies.
Any well seasoned SysAdmin will endorse the following slogan:
If it must run, it runs on Solaris!
