RC5 encryption

Forgive this seemingly naive question, but.... Where does RC5 derive its security from? I know about the ROTL stuff and the key expansion methods (I have the RSA Cryptobytes, HAC, AC, etc.) However, I personally can't see how it would work. I know its supposedly secure because of the data-dependent rotations, and other features. Correct me if I'm wrong, but a ROTL assembler command shifts all bits to the LEFT one and moves the "overflow" bits back into the right end of the register: 0100111011 to 0011101101 if it rotates left by 2 How does this make it hard to decrypt? The round function seems MUCH weaker than IDEA's, CAST's, or even GOST and Blowfish. Vincent Carlos