I bought this lens within a week after it came out; I saw the review on the-digital-picture.com and I was instantly sold.
And then last week I resold it on eBay.
Simply put, there never was a time when I wished I had this lens on my camera instead of any other one. Or perhaps more accurately, anytime I had this lens on my camera, I regretted it.
Ultimately, this is not a useful focal length / aperture combination. I generally use either a 35mm or 50mm prime (with max aperture @ F2 and F1.4, respectively), depending on the situation. If I'm walking around outside on a trip, and want to get architecture shots, then the 35mm. If I'm inside a museum, shop, other building, then the 50mm. I thought that this lens would be a good jack-of-all trades between them -- but as the saying goes, it's the master of none.
First, it doesn't have the intimate perspective that a 50mm provides. Nor does it have the field of view that a 35mm provides. Yes, I realize that the difference seems small, but the difference is quite meaningful in practice.
But more significantly, I realized how much I missed the greater aperture control that the other lenses provided, not to mention the 2 stops of lights indoors (oh Canon, why can't you add in-body stabilization?). And so everytime I took this lens on my camera, I found myself wishing that I had one or the other primes. This one stayed in the bag.
You know, if I really wanted a 2.8 lens at 40mm, I would use a zoom. So this lens is smaller? So what? The body is huge enough. You are using this on a 5D series, at the smallest. When it comes to size, the lens is not the issue. And the 35mm/2 and 50mm/1.8 are only marginally larger. (Though admittedly, this lens is sharper and has a better AF system).
And, no, don't even think about using this on a crop. You'll get a camera system with the absurd field of view of a hypothetical 64mm lens (but with the not so-interesting perspective of the 40mm that it actually is). Such a combination is not useful -- seriously, don't waste your money on this lens if you have a crop, buy the Sigma 30mm 1.4 DC for a little more and be much happier.
And I've heard some people talk about putting this in your pocket. Don't count on it. Canon needs to design new caps -- with the front and rear caps on the lens, the length of the lens is nearly 50% larger, to say nothing if you have a filter on it. If Canon was really innovative, they would have put a protective iris in the lens (like on a point-and-shoot), so that the front cap wasn't necessary. But "innovative" and "Canon" are two words that sadly don't go together any more.
So anyways, after my virtual non-use of this lens for a couple months, I decided to sell it off to fund other purchases, and I don't miss it one bit.
Now, to be fair, I should point out the good parts. It is really quite sharp, compared to Canon's other non-L series primes (in fact, it doesn't do so badly compared to some of their L series). The build quality is really quite solid, and the AF is near silent and very responsive (esp. compared to the obnoxiously buzzing motors of Canon's other cheap primes). So if for some reason you really need a 40mm lens, and you know that you'll never need something faster than 2.8, than this lens is for you. And if you really need the smallest lens possible for your gigantic DSLR, this lens is for you. But would I recommend this to a friend? No way.