A few years ago we had used a subversion controlled documentation folder (great version control), but we ended up with people using their favorite tools to create their docs and hence you needed quite a few viewers or conversion tools to read them all.
In another company most documentation was done on a wiki (easy to learn, great for cross-referencing), but we ended up with people being lazy and not using links to other docs, so gathering all information became tricky.
Currently I am working in a team using Google docs (offering many different tools, quite easy to use, everyone uses the same tools) but I still miss Wiki for the nice cross-referencing abilities.
My impression is there is no fool-proof documentation system around, it's quality always depends on the team' discipline to stick to project-defined how-to's. And if the going gets tough one of the first victims will always be the documentation tasks, whatever system you use.