If you didn't get your invitation for Google Spreadsheets, try this link. It's what I got in my invitation and it can be used by anyone.
You can find some screenshots on Flickr. Most of them show the product works really bad: I tried to import two very simple Excel files, and both had problems. Google Spreadsheets treated them as read-only files. The product is very slow, it's very hard to use (you can barely select a row or a set of columns) and has really basic features.
All in all, this is the most unpolished Google product I've ever tried, it looks like a pre-alpha application.
Philipp Lenssen has a different opinion:
"Google Spreadsheet usability is quite good. As ever so often, this is a Google product with no ads so far (ads might be on the horizon, of course); it’s more a tool than a site, clearly fitting into the growing array of Google OS products. The only thing I really missed so far was right-clicking rows to apply formatting changes or to copy & paste text."
