I have the same lens (on a Canon 7D) and haven't used it for coin photography, so I thought I'd shoot a little side-by-side comparison test since I was curious too. I've only been into this for about a month, but after reading reviews for my first coin photography lens I bought a Canon EF-s 60mm f/2.8 that can achieve 1:1 magnification. By comparison, the best reproduction ratio for the 18-55 is about 1:3.
These first two shots were taken with the 18-55 at 55 mm, 1/30 s f/5.6 ISO 400 at the closest focus (highest magnification). The first is the full-frame image scaled down, and the second is an actual pixels crop of the cross detail on the crown:
These final two shots were taken with the 60mm f/2.8 at closest focus (1:1 magnification, 1/8 s, f/5.6, ISO 400). Again, the first is full-frame and the second is actual pixels:
The subject of these shots is a silver Canadian nickel that is smaller in diameter than a dime. All shots are straight from the camera aside from cropping, down scaling, and a mild sharpening to optimize for screen viewing (EDIT also down-sampled to fit under the forum max image file size of 100K). This should give you an idea of what the 18-55 lens is capable of for coin photography, compared to a good consumer-level macro with a used price tag of about $400. Some of the folks here use bellows/copy stand setups with their DSLRs that yield excellent results at relatively low cost, and can magnify further than 1:1. There are recent threads discussing some of these points. Hope this helps!