The de-facto for encryption with these devices is AES (Advanced Encryption Standard) and is so secure it's used by both government and military. For the USB flash drives on test, the hardware solutions use 256-bit AES while the software solution is 128-bit.
Should your encrypted files be accessed, without the decryption key, the most likely form of assault is a brute-force attack -- systematically attempting every possible combination of key until the correct one is found.
The length of the password you choose, which forms the key for encryption, can have an impact on how hard an encryption is to break: a 128-bit key ideally should be 16 characters long, and 32 characters for a 256-bit key. Most people won't use long passwords because they are hard to remember, so encryption software often fills the gaps with 0s, reducing the effectiveness of the key. Which is why when you're prompted for a password, it's actually better to use a pass [i]phrase[/i] that contains more characters than a password while still being easy to remember. Despite requiring very powerful hardware to crack AES using brute-force, simple passwords are conceivably discoverable.
Using a full 32-character string of random characters, however, is considerably tougher. Just how tough? This is somewhat of a moving target given the acceleration of computer science, but you shouldn't have too much to worry about: it would take some fifty of the world's most powerful supercomputers today working 24/7 up to 3 [i]sexdecillion[/i] years to brute-force crack 256-bit AES. Or, in a numeric prefix we're more familiar with -- approximately 3 trillion trillion trillion trillion years.