I love using Google Docs. When I was a student, I went through a phase where I didn’t have MS Office on my computer because I was writing everything in LaTeX. If Google Docs had been around at the time, my life (and that of everyone else who tried to send me things) would have been much easier. I like the commenting and editing features, I like that several people can edit a document at the same time. I’m not sure how well versioning works, i. e. if it is potentially possible for someone to intentionally or unintentionally destroy a long day’s (or month’s, or year’s) work. I haven’t used it in a professional capacity yet, but I’m thinking of using it to store instructions when the next graduate trainee hits Accessions. It could also be helpful for managing reservations/holds on a spreadsheet.
I’ve participated in a good many wiki projects in my day (*cough*TVTropes*cough*, but some of the classes I took at uni had wikis on the uni websites too, so it was all properly scholarly work), I have a wikipedia account, and yes, they’re useful. The problem is that one person can actually destroy everybody else’s work if they’re so inclined. Or move stuff to a different location so it doesn’t show up where you originally put it.