Though it may appear on the surface that there are a lot of varied online storage offerings available, when you dig into them, it becomes easy to categorize them.
First are the online focused storage/sharing sites like Box.net and Omnidrive, both of which stress sharing and online collaboration features. Of the two, Box.net is clearly superior in that it does everything that Omnidrive does, but better. Box.net's add-ons and widget capabilities are really slick and give it a clear win.
Then there are hybrid desktop/online storage/backup and collaboration services like Xdrive, MediaMax and IDrive, which aim to combine multiple elements. Though MediaMax offered the most for free, the quality of service was unsatisfactory. IDrive has a solid backup approach, but in terms of the online sharing functionality, it falls behind Xdrive. With its mix of backup and online sharing, Xdrive might well be the best in this category, though it's not a clear choice.
Finally, there is the category of pure backup services; these are solutions that are only concerned with backing up what you've got on your PC and securing it in their online storage system. This is the domain of Carbonite and Mozy. Carbonite is slick, but you can't restore a file from the online interface - in fact, you can't even see what you've got from it. In an emergency, when you've lost stuff and you just want it back, you may not want be able to reinstall the client software; this could be a really serious problem.
Mozy doesn't have the simplicity of Carbonite when it comes to actually identifying what files are being backed up, but it offers a superior array of functionality. With its multi-layered bandwidth and CPU throttling, combined with multiple options for restoration (including the online interface), Mozy has a leg up on Carbonite.
In the final analysis, online storage is a personal choice, be it for sharing and collaboration or for pure backup. There may well be an option or a selection that one particular service may offer that you really want. That's why the free trials with no credit card information required are so important. There's no risk in trying them out, and if you don't already have an offsite backup plan, the only risk you're taking is not actually backing up online...