I had the EF-S 17-55 f2.8 IS for the crop bodies before purchasing the 40 2.8. Optically, the 40 didn't offer anything that the 17-55 already gave me -- except it's compact size. I do feel less conspicuous with the 40 and for me, the difference between the 40 and the 35 in focal length is negligible.
It really comes down to that extra stop.
I shoot a lot of figure skating events. The most challenging are ice shows where I'm contending with spotlights for performance shots and flashlights for backstage photos. Two years ago I rediscovered my old 50 1.8 for backstage photos with a borrowed T2i. At the time, my main lenses were the 3.5-5.6 zooms. The 50 made these backstage photos possible.
I then upgraded by adding the 60D, 35 f2, 70-200 f2.8, 17-55 f2.8, and 7D (in that order) before last year's ice show. I expected to be using the 35 extensively for behind the scenes shots at ISO 6400. I actually found the 17-55 more useful do the IS and quicker focusing.
More recently, I shot a school event at a bowling alley with "cosmic bowling" (as in very little light). All I took was the 7D and the 35 2.0. My keeper rate was very low and I attribute this to problems locking in on focus with the 35. I had better success at a similar event with my 60D and the 17-55 f2.8 IS.
My conclusion is that the 35 on crop when shooting ISO 6400, f2.0, and shutter speed slower than 1/100 is a hit-or-miss thing. I'm better off with the 17-55. But, if there's enough light to shoot at 1/200 of faster, the 35 may give you an edge in stopping action.
My solution to the low light backstage challenge was to add the 5D3 to my kit. There's another ice show in a few weeks and I plan on using the 35 on the 5D3 for that extra stop and comparing this to the 40 (and likely the 50 1.
As for overall value, my 40 is used a lot more than the 35. Candidly, I'm still debating the real value that the 35 offers. For very low light on crop, it has been of little benefit over the 17-55. I have higher expectations with the 5D3. But then, the 5D3 is so much better in low light, I may still prefer the 40 for its sharp corner-to-corner performance. I'm not adding any more lenses until I get more experience in low light with the 5D3, but FWIW, if I didn't have the old 35, I'd rather put that $300 toward the new 35 2.0 IS (or the 35 1.4).