What makes these lenses so great are their high quality, simplicity and size. Photozone/DXOMARK etc all show in sharpness they often match the L primes at same aperture.
Many of us lug around big f/2.8 zoom lenses because quite simply f/4 doesn't cut it sometimes. It is rare, though, that ~f/2.8 + IS doesn't cut it. There may be special applications where this is the case, but with IS most of those situations are negated.
However, when this latest series of Canon primes are shot wide open, their sharpness and quality generally MATCH (and sometimes exceed) the 1.2-1.4 L prime lenses that cost 3x as much and weigh 3x as much. Only thing wide open is vingetting, but that is easily removed if it looks bad in a shot. Otherwise they are sharp across the frame, fast focusing, have real manual focusing, light, and well built. No they aren't weatherproofed, but hey if you want to stand out in the rain just use one of those big zooms with weatherproofing
As an example, the 35mm f/2 IS USM for most people's uses would be overall superior to the 35mm f/1.4L - the f/2 IS actually has better bokeh quality and is lighter/smaller than the L. Obviously the f/2 IS does not have 1.4, but whether the 1.4 is worth the increased cost, increased weight, increased size and worse bokeh is the question; FWIW, DXOMARK ranks the 35mm f/2 IS above the 35mm f/1.4L.
So, when you have a few big f/2.8 L zooms, these new IS lenses are great to have as backups to the zooms or simply when you want a lightweight companion with you. They compliment the f/2.8 L zooms very well and in some cases exceed them. Thus, a great addition overall to your kit!! You could build and carry a whole kit of these for the price/size of one L prime, and probably get about the same results in image quality - very cool.
24mm IS + 35mm IS + 50mm IS + 85mm IS + 135mm IS = ~$2500
24mmL + 35mmL + 50mmL + 85mmL + 135mmL = ~$7500
Given the quality exhibited by the IS lenses, is it really worth spending $5000 more on the L versions and carrying all that extra weight/size around? For some purposes, it may be worth having one of the L lenses, like maybe a nighttime star photographer might want a 24mm f/1.4 for the low coma at f/2.8 (though the 24-70 II zoom @ f/2.8 can do the same)... Or a portrait photographer may want the 85mm 1.2 for its speed... But for most applications I would think a kit of the IS lenses just makes a lot more sense - and if you pair with f/2.8 L zooms you have all bases covered IMO.