Personally, I've been using Dropbox since 2008, and have had mostly positive experiences. I use it carefully, in the sense that I try to be aware that my data is being managed by a commercial organisation operating out of a different jurisdiction.
Now that Dropbox has proved itself there's a bunch of competitors like MS Sky Drive, Box.net, and so on. I'm interested in making TW work well with all of them. Now that Dropbox has led the way with a JS API for manipulating files, I'd expect their competitors to provide broadly similar offerings to remain competitive. So, right now, I'm focussing on getting a good experience with Dropbox for both classic TiddlyWiki and TiddlyWiki5, but I'm trying to position the work as being more generic ("TiddlyWiki in the Sky" referring to the concept of storing ones TiddlyWiki's on a cloud service, and the "...for Dropbox" bit referring to a specific instantiation for Dropbox).
The concerns about the trustability of ones cloud provider can be mitigated by encrypting and decrypting data in the browser, and only ever transmitting and storing encrypted data. In fact, it would be possible to create an encrypted TiddlyWiki and leave it in plain sight, confident that only those with the credentials will be able to decrypt it. So, TW5 uses the Stanford JavaScript Crypto Library for integrated encryption.
All of this is a long way from the old world of keeping TiddlyWikis on a USB stick in ones pocket, but it buys us something profoundly useful: the ability to work on any internet connected browser, whether a smartphone or a desktop.
Cheers
Jeremy