Compressible file system has been a great benefit for archiving large numbers of small text files.
No, very stable.
No, not really an issue in our environment.
Fairly straightforward. Storage seems to be the most complicated part when dealing with physical servers.
This was a long time ago. We considered Linux but wanted the vendor support offered by Sun at the time with both hardware and software, because we didn't have expertise in the area.
Nowadays, I don't think Solaris has a lot of advantages over less expensive options. I do love ZFS and have found it to be very stable. I don't have much experience with it in other distributions but some early attempts with fuse were not stable. That was a while ago, so I bet you can get a stable release of something with ZFS.