I've always viewed ICs as being solid 4.5 star hotels. Definitely not within the same category as true 5-star hotels as others have mentioned, but reliably better than your run of the mill Sheraton.
Putting it on bar with JWM/Conrad seems about right.
Re: comments above, MO is definitely 5-star except for maybe in Malaysia, and I would put the Luxury Collection in the same boat as IC.