Now with Release 6.1, the Secure Server 3.1 is included with the Professional Version. Version 2.0 was easy to install and setup, but the documentation was somewhat spartan. I have worked with Enterprise on Solaris. The Java administration tool makes managing this beast very easy. Documentation is outstanding.
Pricing is concurrent connection based.
Despite rumors to the contrary, I also use Microsoft products. Internet Information Server is available with NT Server. It is easy to set up and maintain. Remote administration can be a hassle. Well supported and documented.