The only problem is that it's all hosted by Google, subject to change at any moment. If it has the functionality (I've never used it), definitely save anything you regard as even slightly important that you create with it onto your own computer or flash drive (if travelling about).
Many thanks for your reply. I have been using gmail since its launch and am very happy with it.
I've now tried this spreadsheet - it imports very well, all the functions and formulae are carried over. I will keep back ups, though, as a precaution.
If you don't need it to be portable, I can strongly recommend OpenOffice. It's free, and about 95% compatible with Microsoft Office. It even does some things that Microsoft's version doesn't do.
On a sidenote, if you use an rss feed reader, then check out Google's Reader. They've just updated the interface, and it's now very usable.