I've only skimmed the site, but in terms of pure content management the ACS is lacking. However, aD's putting resources into filling that hole in the toolkit so this won't be true for long. If you can't wait, Manila might fit your needs.
It doesn't appear to be built on top of an RDBMS, or at least doesn't mention this on the summary page. If not, you may be more tied into their code than you might be with the ACS - in other words, since the ACS is built on top of a standard RDBMS engine the contents of the database are available to you as a web programmer. That means you can write any kind of skin you like on top of the datamodel if you don't like the way the ACS does something, and as long as you maintain the integrity of the database, your custom pages will work fine with the rest of the toolkit. If Manila sits on top of a custom flat-file storage system, it probably isn't as flexible in terms of being customized by the programmer.
But that's not what they're selling, anyway. They're aimed at out-of-the-box solutions, while the ACS is aimed more towards being a toolkit for people to build high-end and highly customized web sites. Depending on what you're looking for, this alone may be a determining factor.
ACS 4.0 will be a big step forward, but is going to take a while to roll out in both ACS Classic and OpenACS form. Since it will have a package manager, it will be easier to cobble together a custom site from pieces without doing custom programming, or fixing all those missing or buggy user group customizations and stuff that only incompletely exist in today's modules - all modules will play nice with the core when they're rewritten to the package manager spec.
But ... that's the future.
Lastly ... Manilla's not free, while OpenACS is. Download a copy of the ACS and play around and decide for yourself if it meets your needs!
