What do you like best?
In a nutshell, ZFS is often the reason that we select Solaris to deploy in the enterprise, even in heterogeneous environments where numerous Linux and BSD machinery is running, the advantages of making storage facilities available that incorporates the snapshotting features of Oracle Solaris is compelling, even in a landscape where Btrfs is increasingly common under Linux.
In conjunction with the marriage of Oracle hardware that never needs to be downed for the replacement of memory, CPUs, etc., Solaris offers metrics of uptime in the data center that exceeds most other Unices by virtue of being able to continue operating even during bare metal upgrades and maintenance.
What do you dislike?
With respect to the native file system ZFS, yes, it is available for Linux, but due to Oracle's licensing restrictions, the way most enterprises choose to deploy it is in user space using FUSE, since it can't by default be *distributed* with out of the box support in Linux.
That, and the fact that Btrfs is now considered stable and production ready (as evidenced by SuSE's adoption of it as the default FS which began with SLES), the deployment of snapshot on write filesystems is still primarily dominated by Linux in the enterprise.
One also has to bear in mind that even going back to earlier versions of SunOS, Solaris is a bit of a different creature so there is a learning curve to performing the same systems administration functions in Solaris (i.e., "ps -e") as opposed to the way most Linux admins are used to with Bash. This necessitates addtional thought and preparation of cross-compatibility when deploying shell scripts within environments that incorporate a mixture of BSD, Linux, and Solaris operating systems on their machinery.
Recommendations to others considering the product
Make sure you know why you want to adopt this 'particular' UNIX - If you don't know why you want or need Solaris, then you're probably better off deciding upon one of the xBSDs or a Linux distro.
What business problems are you solving with the product? What benefits have you realized?
One particular plus (or minus, depending upon your perspective), is that there is a certain level of vendor lock-in - customers can't just call up a fledgling IT support company down the street. To a certain degree, this is even more pronounced with the plethora of mediocre, so-called MSPs that can only tend to Windows environments, or only feel comfortable trying to manage UNIX machines from a GUI.
We often find that our new customers come to us out of frustration of having these sometimes less than competent providers tend to their IT needs, and when serious computing requirements are involved, realize that spending more on an hourly basis actually translates to an even greater return on their investment where ROI is concerned.