Excel Web App can handle large spreadsheets, with plenty of data. It's an easy way to share data with colleagues, and to explore large amounts of information collaboratively. Text is clear, and it has the familiar Excel look-and-feel.
You can't create Excel charts online, but you can upload them — and because they're dynamic objects, you also see the online version change as you alter the underlying data.
The only way you'll see if someone else is working on a shared spreadsheet at the same time as you is when they open the document. Then, you're presented with a little yellow pop-up in the corner of the file.
Excel 2010 has plenty of tools for handling conditional formatting of spreadsheet content, with in-cell icons and colours to add information to your content. The same features are available in Excel Web App, although you can't edit them online.
There are plenty more editing tools in PowerPoint Web App. You can create new slideshows, or edit existing slides, adding fresh content with familiar tools.
One of PowerPoint's more useful features is SmartArt, which simplifies creating simple diagrams for your presentations. It's even more useful online, letting you quickly add graphical content when you don't have access to the full range of the desktop PowerPoint's design tools.
If you want to share documents with colleagues, you'll have to use SkyDrive's permission system. It's not as fully featured as the one in Windows, but you can at least control who can see and who can edit a document.
If you're used to the dynamic thumbnails in Windows, you'll find the SkyDrive file view a little primitive. However, it's clear and easy to use.
Microsoft is still tweaking the Technical Preview as it adds more and more users, so be prepared for the occasional outage.