Since Basecamp has added writeboards, I haven't been using Writely much. That is not at all a knock against the app, but since I am so invested in Basecamp for managing projects I tend to use the features there if possible.

However, Writely is moving ahead with lots of new features which makes the offering more attractive. In particular, they have added PDF support:

[from Writely Blog

# Save as PDF.
Look for 'Save as PDF' on the Action menu in the editor. Note: this is our first to-be-premium feature, meaning it will be part of a paid subsription service once we come out of beta.

# MUCH better collaboration collision handling.
We've fixed some problems and added a pop-up that lets you retrieve your lost changes.

# More bug fixes.
Find & replace, the @ sign for Int'l keyboards.

# More localized FAQs.
These are now available as a drop-down from the Help Center and FAQ page. We've added Traditional Chinese, Turkish and Vietnamese. Thank you, fabulous volunteer translators!

# The beta meter.
Help us decide when we're ready to come out of beta!

As Michael Arrignton points out, creating PDFs is of high value, and, on Windows, cannot be done in Word unless you've purchased the Adobe PDF software. On my Mac, that's built into the operating system.

As soon as someone creates a replacement for Excel, I can drop Microsoft Office altogether. The Numsum experiment is too weak, and Jotspot Tracker is really just a way to display spreadsheets. I hope Writely (or someone) is working on this.