I happen to work for a Linux company of German origin, and I was talking with one of the fellows at SlackWare a while back. We were discussing t-shirt ideas for Linux companies. He mentioned that they had been trying to talk marketing into creating t-shirts that say, "I can't believe it's not UNIX!!!" This hasn't happened yet, but I thought this was insanely humorous. I have this appended to my .signature file now.
Linux is not UNIX, and many people fail to notice this simple fact. Linux is UNIX-like, but not UNIX-based, as development of Linux began from a clean code base. I would say that Linux, in it's current state today, is a very well maintained, well developed, and well supported (through the community) operating system. It has many features today that are not found in commercial UNIX's. It is also important to note that there are literally hundreds of "flavors" of commercial UNIX existing today. Many of the different UNIX's are very well suited to do specific tasks. Some have better Logical Volume Managers, some do a better job of managed accounting systems, etc.
Linux is well liked, because it does a little bit of nearly everything, and it is free and easy to develop for. Linux is not the solution for every company or person, in every situation, but neither is IRIX, HPUX, AIX, etc. As semprini mentioned, a good deal of it comes down to personal preferences and experiences.
Thank you all for listening to my rant.