As far as I can see the IMC Rating is to the IR like the PPL is to the CPL ie it covers the same ground but the expected standard is a little lower.
Contrary to what has been said here- and I've heard the same thing elsewhere - the IMC Rating does allow you to set off and fly all day in IMC perfectly safely. It is not a get-you-out-of trouble. That is part of the CPL training. You should be able to enter and maintain IMC and carry out a let-down under the help and guidance of ATC.
I believe you can get one very cheaply in Florida and it used to mean you had to do less hours on the IR although that may well have changed with the advent of JAR.
One advantage is if you pass this you should feel a lot more confident about the IR which is perhaps the most stressfull test on your route to the ATPL.