I have the most experience with Frontier v5.0, which is the last free version of UserLand's software. Frontier is the precursor to Manila.
Basically, Manila is based on a home-grown language that I would describe as not particularly strong, but it has a big library of verbs (in TCL we call these procs) many of which are designed for HTML generation and the like.
It has some nice features, but it is limited for a number of reasons:
1. no Unix version
2. Mac and Windows versions not particularly robust
3. it uses its own "object database", which is both the source of its nice features and its greatest curse, since it is not that good a database
4. Something, somewhere, leaks memory.
5. Dave Winer is a nutjob.
OK, some things that OpenACS does better than Manila:
1. Reliability - runs on unix-like OSes
2. Real DBMS access
3. Easier to extend templating system - TCL is much easier to understand than Dave's eccentric homebrew language, which combines elements of HyperTalk, Pascal, C, and AppleScript
4. Easier to back up
5. Offers versioning support
6. More robust user handling (for cases where multiple users are updating content)
Manila is, however, much easier to get going.
