It depends what you plan to store in the cloud. As soon as you've got financial or personal data, Wuala to me is the only one I'd trust, as it is hosted in Europe between CH, DE and FR, and encryption is done on the device, not in the cloud. This protects you from the US Government snooping under the Patriot Act. Furthermore, as it is NOT a web service that can be viewed in any browser, you have a chance of being better protected from other snooping. You can use a Browser where Wuala is not installed, but it creates a Java App on the desktop to encrypt and decrypt files. This could be a problem in a tied down Office Desktop Environment.
I use it for some photos (I need unlimited upload to send them all up) and am happy enough with it, although I must admit iOS integration (as with any iOS cloud app) is painful, as the file system is separate from iWork, and for iPhotos, you will need to duplicate your data. For this reason, I am looking forward to Wuala for Windows 8 Metro, as I will be moving my administrative world to Windows 8, and leave the iPad and probably my beloved Mac for what it was intended for, games and entertainment. (I know I've probably just started World War III saying that according to the Apple Fan Boys - and I've been a dedicated Mac User for 7 years)
Inviting friends and family to access files with Wuala is tricky, as it is a two step process, and the second step I have found is too much effort for family and friends (ie accepting the request in the client to join a specific photo group) because it is a Document and Record Management System (albeit a simple one), which from my experience always freaks people out.
I can't say I've heard of justcloud. Where is it hosted and how is the data being managed is the question I asked myself when selecting a service, and for anybody going to the cloud in my opinion is what they should be asking themselves. (I am a former Information Architect for a government agency, so I am supremely paranoid).