I picked acs over 3 years ago after reading an Arsdigita quote "ACS will save you 1 million in upfront development costs" or something like that. I had run aolserver/informix before that but when my Sun died I switched to NT/access/asp or NAA. At the time I owned cigar.com, t-shirts.com and worked for an ad agency. My biggest problem with asp is you can't write reusable code so I started looking around and found ACS.
At that time there was really nothing else but now servlets and most likely .Net have similar base functionality. I don't know much about .Net but I currently use servlets and I've used PHP.
Linux, Apache, Mysql and PHP (LAMP) users might have some interest. From a technical standpoint LAMP is about the same as NAA. I personally don't like Mysql because I don't understand it's liecense agreement. The great thing about LAMP is there it's easy to install and get something to work. You can also host it anywhere cheap. The later is a big barrier to OpenACS.
Jboss is also a very interesting project and if I were picking now I might pick Jboss. The problem with java is simple things are not simple. The pluses are good XML handling and it's pretty fast. The minuses are it's compiled (big minus) and it's strongly typed (bigger minus IMHO).
So what was my point? I still think OpenACS is the best blend of simple and scalable. The api-doc, database api, packages and templating system are it's best features. It comes with a set of packages that serve both as real apps plus examples. It's perfect for a small web development shop that hosts many sites that need more than just HTML.
