If you crypt, Dropbox is fine. People need to use encryption. Every popular computer language has encryption routines, scroll through the source code until you find something accessible, twiddle something to personalize it while keeping it functional, perhaps convince yourself it will remain secure, etc, of course be cautious about that. Or simply, there's double encryption, fold it again. Know big 100 meg, gigabyte file size encryption, becomes vulnerable. Wikipedia is the best general crypto introduction I've see.
The main selling point of Dropbox is cross-platform support for umpteen platforms, so you'd need to find an encryption tool that will work on all platforms; say bye to iOS...
Don't mess with publicly-vetted crypto code - you're far more likely to introduce a weakness. Instead, just follow the documentation and use it correctly.
