In the last year I used OpenACS mainly to build an almost complete ERP system.
I used OpenACS in various kinds of applications and I can't think of any of them for wich I wouldn't recommend using it to get the job done.
I find Simon's list of arguments almost identical to that I wanted to write down and so I limit myself to emphasize the wonderful database integration (also thanks to Aolserver) and the templating system.
The one thing that I don't like is the implementation of the service contracts, wich I find too complicated. I think that they are modeled after the WSDL, but personally I find much more easy to deploy a web service via soap-gateway instead of using the service contracts.