• It’s responsive. For anyone who has used Google Spreadsheets, this is a key point. There is not a lot of, if any, time spent waiting for the program to catch up.
• Writely’s built-in collaboration capabilities make it a suitable alternative to other versioning software for use during the beginning stages of editing, when changes by different individuals are being made often. At first creation or any time further along, you can decide to invite others to collaborate on the document(s), allowing them write, read, or both priveledges.
• Past revisions can be viewed individually or compared to each other, with the capability of reverting to and saving a version of the document that existed before any unwanted changes were made. There is also the option for having an RSS feed of the document changes, though I’ve not used this.
• If at any point it is decided that the document should be saved locally to a more secure location, Writely allows you to save in any of these formats: RTF, Word, OpenOffice, HTML, or PDF.

In conclusion, Writely seems like a capable resource for collaborating on documents, with many features that extend way beyond what was described above. Although Google says that user’s documents are very secure, it might be wise–the beta meter is at 62%–to beware the use of Writely for documents that contain any private data. Still this web-application is definitely something that should be tried out.

Here’s a couple reviews:

Cnet
About.com