However, iCloud does not have a DropBox like document storage feature. Apps can store documents in iCloud, if the developer adds that feature, but it's only for that app. There is no general access file storage area in iCloud.
I use BoxCryptor (www.boxcryptor.com) and/or TrueCrypt containers to encrypt items I want encrypted in the cloud. I have a specific folder that has all of the encrypted items in it using BoxCryptor. It also has mobile apps used to view encrypted items in those folders for on-the-go usage. Very easy to use and set up. TrueCrypt I have used in a large variety of ways. There is an issue with it and services like DropBox and SkyDrive where it must change the whole container instead of just the updated file. But, I use BC since I only want specific folders encrypted. Works like a charm and it's Free! I purchased the unlimited version so it will also encrypt the filenames.
but that may just be because they are more forthcoming. On the other hand, Box has been targeted at businesses all along, so it's had to take security seriously all along. They need the trust of the companies that depend on them.
In my opinion both are secure enough for most people and most uses. I would hesitate to save critical financial information or company secrets on either. If I did, I would encrypt the files using my own utility, as Todd M suggests. Their encryption plus your own 256 bit encryption makes for a very secure file.