The consensus seems to be that yes, it is.
Binary compatibility was much improved in 2.3. That will help transition somewhat. The performance of 2.3 is adequate, though some parts of the system are still slower than SunOS 4.1.x. Solaris 2.3 is much more stable on MP machines than 2.2. The Solaris 2.3 version of OpenWindows is much faster and much more stable than the versions shipped with SunOS 4.1.x.
Solaris 2.1 and earlier should really be avoided. Solaris 2.2 should be avoided too, but some people need to stick to it until some applications get ported (2.2 is the last release with NeWS).
Solaris 2.3 still has some problems on high-end MP systems with large numbers of interactive users. Solaris 2.4 and Solaris 2.5 have delivered increasingly more stable and more scalable multi processing.