I tried to resist the urge to buy this but ended up pulling the trigger, and all I can say is WOW so far. The build quality is amazing, better than any lens I own. It's a bit deceptively heavy due to the metal, but it feels very balanced on my 6D. Seems extremely sharp, even wide open (not sure about the edges, but really, how often do you shoot at 1.8 and need tack sharp edges?). It's also the first lens I've ever bought that focused dead on without any microadjustment. The thing I really noticed though is that 45mm seems like a great focal length for street photography - I've kind of been on the fence about whether 35 or 50 is better for that, and I've mainly stuck with the 35 F2 IS because the IS is so useful for night shooting. 45mm seems slightly more general purpose than 50mm, and I think it's gonna be the perfect compliment to my 16-35mm F4 IS in my travel kit.
The 2 downsides I noticed are the same ones everybody has pointed out - 1) chromatic aberration is a bit high, esp at 1.8. Should be correctable once DXO/Lightroom release plugins, but it is an issue. 2) AF speed is definitely on the slow side. If I had to pick between accuracy and speed, I'd choose accuracy, but it's definitely noticeably slower than my 35mm F2 IS.
Overall the lens is amazing, it's what I've been waiting for as far as a great general purpose normal prime (wake up, Canon). With a fast normal image stabilized zoom, I think I can finally sell my 24-70 VC, since there are few remaining situations I can imagine wanting to carry it around.
