My thoughts
If you are going to compare IS to a tripod, then the tripod wins hands down. A tripod will help you slow down and consider your shot, although it can be cumbersome to haul around and set up. IS wasn't and still isn't an option for the tools used by arguably the best landscape photographer around: Ansel Adams. Adams hauled a huge view camera around with a tripod to record his images. Most of Canons lenses that have a non IS and IS version, the non IS version is usually sharper.
IS is for situations where you can't use a tripod, or if you don't want to carry a tripod around. I own one IS lens, the 28-135, I bought it specifically for use in aerial photography, to tone down the vibrations of the airplane. All of my other lenses are non IS and I lug a tripod and remote release to get the sharpest photos possible.