They are a second tier grading service, one rung below NGC/PCGS/ANACS and better known for their grading of ancients than modern coins. I've seen a few coins in their slabs and the grading is OK. IMHO you're best evaluating the coin on it's merits rather than paying to much attention to what it says on the holder.
ICG, ICCS, ACGS, CGUK are all second tier IMHO and not too bad. Not too bad in that there may be some correlation between the quality of the coin and the grade on the coin. Slabs with letters like NNC, NNS, SEGS, SGS don't so much occupy a tier, but live in the basement. Avoid.
Not sure of the veracity, nor do I have any firsthand experience with them, but I've read at a number of sources that ICG tends to grade too high these days. I just stick with NGC & PCGS..