So, we're using Gmail/Google Apps at my new job. The doc feature makes collaboration a breeze. It has issues though.
1. There is no lossless way to backup documents. This shouldn't be a problem because Google shouldn't lose stuff. However, some of us were using our personal gmail accounts until the new system was available. If you only had a few docs, it's easy to share the docs between accounts and make a copy in the new account. You lose all revision history in the new doc.
If you have lots of documents, this is difficult to do. There's a download tool that will allow you download the docs in a specific collection, but only in a different format. Formatting is lost. So, it's not lossless.
2. There are only a few specific styles and you can't add any new ones. You can change the pre-defined ones, but that's of limited use. In previous versions, you had the opportunity to change styles via the html input capability, but that's gone now.
3. No ability to reference another doc and stitch multiple docs together at export to pdf time.
4. No image caption capability. This isn't a big deal. Insert a 1 cell table and put text at the bottom.
Because the range of formatting options are limited, it would be really nice if GoogleDocs provided a basic DocBook or even LaTex export capability. It would make it easier to integrate the documents into CM repository. It would also allow you to create your own formatting and combine documents as needed.
The strength of GoogleDocs is it's online collaboration capability. You don't to pass around word documents and worry about who has what revision. I don't know if Sharepoint makes this easier now. Probably not since it's just a storage location.