Thinking about why Solaris has failed here
First off, as I rambled yesterday, to say that Solaris has failed here is not to say that our use of Solaris has been a failure (or that our Solaris machines have been); our fileservers are stable and run (generally) without problems and work. But at the same time it's clear that Solaris has failed to catch on here. The only strong reason our fileservers run Solaris is ZFS, and we've not even running anything close to a current version of Solarism at that (cf); at this point our fileservers are less servers and more sealed black-box appliances. We've never had any interest in using Solaris for anything else and we're not very enthused about Solaris even for ZFS; to put it one way, we pretty much use ZFS despite Solaris not because of it.
(There's good alternatives to Solaris for ZFS nowadays, but as it is our fileservers work and switching just to get away from Solaris seems uncompelling. Or even insane.)
Recently I've been thinking about the question of why Solaris failed here. At one level (as Solaris fans will tell you) Solaris is not unattractive; it has a number of nice features, including ZFS. But it clearly did not hit it off with us. Why? I think there are two major reasons.
The first is that Solaris is, frankly, somewhat erratic, flaky and buggy; it's not the kind of simple and solid system that, say, OpenBSD is. OpenBSD doesn't do very much but what it does just works and works reliably, which makes it a pleasure to use within its areas. Our Solaris machines are pretty stable but getting them there took a significant amount of work and local invention, and the reason they're stable is that we don't change anything on them and we don't ask them to do things that we know are problematic. Since Solaris has problems where we have looked, it probably also has problems where we haven't had to look yet.
The second is that Solaris doesn't have the package availability of
many Linux distributions or FreeBSD, where you have a wide selection
of relatively current open source software that is maintained by the
vendor. There are outside efforts to provide open source packages for
Solaris (and we use one of them), but outside efforts can disappear
(and are not official) and so are intrinsically less trustworthy and
confidence inducing than something the vendor itself supports. Unless
Ubuntu absolutely explodes, a relatively current Apache is always only
going to be an
apt-get away and someone else will always handle
security fixes for it. This is not something we could ever say about
(Part of the problem of third-party packaging is that Solaris itself ships with any number of very old versions of things as official packages. All of the resulting options are bad ones.)
A significant contributing factor is that Solaris is simply not that pleasant to administer, partly because it is almost totally without modern niceties and partly because what it has in the way of administrative tools are often terribly bad (try to tell me with a straight face that Solaris 10's patching system is at all tolerable). Solaris administration is full of things that might have been good ideas if they were competently implemented but, well, they weren't.
(I understand that some of this has changed in modern Solaris, but by this point that's too little, too late (for us).)
The combination of these attributes in a single OS means there's nothing here to really feel enthused about using Solaris for. It lacks a speciality and a niche that it can really dominate in the way that OpenBSD dominates firewalls and related networking stuff, and it lacks the wide package availability (and administrative convenience) that would make it easy for us to deploy a Solaris machine to do <X> for some <X> like 'provide IMAP'.
(Our directly user accessible machines are essentially forced to run Linux (and Ubuntu at that), but we have plenty of servers that users don't log in to and that could theoretically run almost anything.)
As always, everyone's circumstances are different. For instance, if you build your entire software stack from source yourself in order to maintain full control over it you don't care in the least about Solaris's lack of prepackaged open source software; if it existed, you wouldn't use it anyways.