When i first started looking at FTP servers back in 1999 or so, i remember two major players, Serv-U and WarFTP Daemon. Serv-U was much easier to work with so i went with it and I have never looked back.
Times have changed though and today there are so many other pieces of software that do the same job...free and not free. But even today, Serv-U holds its own.

From the days of version 2.5k and now, there have been major GUI overhauls, for the better.
Im so used to the Serv-U gui (version 3 and later) that i really dont feel like changing to another server to, forcing myself to learn a new GUI. Its all menu driven and feels like the windows registry to an extent...i like it. But you have to note though, i havent looke dat other servers, so i really cant compare.

Feature wise...Im not really a heavy duty network administrator who needs to analyze logs for every security breah or something (nor do i host an anonymous FTP for free hosting), so I probably only use like 50% of the features Serv-U offers...if that.
But those 50% i do use serves me very well...usually with software i find that i really need a feature because i like to do certain things, but with Serv-U i havent really felt that way. It does everything I need to do.

Ease of use...this is a tough one. There are two types of people in the computer world, the inexperienced and the experienced. I fall into the second category. And from my experience, Im turned off by wizards (and im sure thats the case with most experienced users) which help automate the process. But in the case of Serv-U, if you don't use the wizard (which Serv-U suggests you do NOT do) you are pretty much up sh*ts creek. There is one step in the wizard process that is so vital to getting the server up and running, that its important to run the wizard. But once you get thru that one step everything else can be done without the assistance of a wizard. So with the wizard, the server is pretty easy to use. Without the wizard, jeez forget about it. But serv-u knows this and that is why they implemented the wizard.

Value...its an FTP server that can integrate as a system service...thats a lot of value for me...i like it that way, not having to worry about it showing up in MSConfig or the sort. Its a great FTP server which gets the job done...so its of high value to me.

Overall rating: excellent...it does its job, and very well i might note. Ive been using it for over 5 years and have been very pleased with it. I really cant think of anything i find BAD about the software, because ive just been so satisfied.
If you dont mind paying the steep fee, you will get probably the best FTP server available.

There are definitely free alternatives...i just havent tried them out and really dont plan to.