My only disappointment with the latest mid-tier primes, is that while they gained IS, they seem to roll in at F2.8 instead of F2, or even F1.8. I'm not sure how IS works, I'm guessing heavier larger glass for those wider apertures would be harder to move for their current IS mechanics, and therefore to ensure quality they made them lighter, coming in at F2.8. But I don't know that at all, other than a gut feeling which could be completely wrong.
I really want an EF 20mm F2 IS. Not a F2.8 IS. Granted, the current 20mm is F2.8 with no IS.
Then again, I really want 24mm F1.4, but I don't want to buy `L just to get it. And I'm not quite ready to go to a manual lens for it (Samyang 24 F1.4).
Again, the new IS primes replace their old screw driven counter parts, I don't think the f/2.8 had anything more to do with it than just them making the lenses available at the same aperture as the one they replaced. The 24/2.8, 28/2.8 and 35/2 were all old screw drive entry level primes, all were replaced by updated (still entry level, albeit more expensive then before) IS primes of the same aperture value.
Also, Sigma has a 24/1.8 out right now for around $500 and it's decent, but not nearly as good as the 24mm IS from Canon (which is about the same price).