Tresorit's biggest selling point is the strong security. Your files and folders are encrypted before they're uploaded to the cloud.
The Windows software (Mac, Android, and iOS versions coming before June) is also really easy to use. You can select any folder to be synced (as a "Tresor"), share folders with other users, and also adjust permissions of shared folders by user.
On the downside, there's no web-based access for your files, people you share folders with will also need the software installed, and it takes a few seconds longer for your synced files to appear on another computer (probably because of the encryption that has to happen first). Other Dropbox alternatives with client-side encryption, SpiderOak and Wuala, have more features, but Tresorit's interface is a lot more user-friendly. And it's hard to beat 50GB of free encrypted space.
If you want to try it out, you'll need to register via the link below to get the 50 gigs free. This offer is good until May 20.