RC5 encryption

[vcarlos35 at juno.com]

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