I've had a few problems with Google's online spreadsheet. But I still think it's a fantastic product, primarily due to its collaboration system: multiple people can work on a spreadsheet at the same time and text chat about their work while they do.
But making the transition from Excel to Google is not as smooth as you might like. While nobody expects an online spreadsheet to have all the features of a mature application like Excel, I often find myself frustrated with Google's limitations. For example, there is no charting function. Whoever heard of a spreadsheet that doesn't let you make graphs? There's also no way to freeze title rows or columns, so on a big spreadsheet it's easy to get lost.
I expect Google will layer in these and other features eventually, but if you can't wait for them, there are alternatives. For example, EditGrid, is a very slick browser-based spreadsheet that has the two features I mentioned above, as well as a few other niceties, such as a permalink for each spreadsheet and several options for embedding and linking your sheets into other pages. You also can use live data (such as stock quotes or data from other Web pages) in your spreadsheets. NetVibes works with EditGrid, so you can create your own data dashboard and embed it into your start page. That's pretty sweet.
Like Google, multiple users can work on an EditGrid sheet at the same time. You can invite collaborators who are EditGrid users, or you can send a password to anyone not registered on EditGrid to give them access. The problem is there's no way to invite someone to become an EditGrid user via the system, so a lot of users will probably rely on the insecure send-a-password scheme when they want to add collaborators. And there's no spreadsheet chat window, as there is in Google.
EditGrid's embedding and publishing features make it a very good choice for people who want to include their spreadsheets on other pages. As a standalone spreadsheet, it also has a few more useful features than Google. But Google's spreadsheet is easier to use, and its invitation and collaboration system is slightly better.