The 5Dc becomes a bit noisy at ISO3200 (it's a boost ISO anyway, which is more or less the same as just increasing the exposure by 1 stop in PP), but even then, it retains a crap ton of detail at high ISOs.
I wouldn't hesitate making A2 sized prints from even ISO1600 shots, let alone lower ISOs.
The big fat pixels help in that regard, especially if you shoot RAW. Even when there is noise, there really isn't a lot of lost detail.
The only thing I hate about mine is the weight, and that's largely because I've spoiled myself with my X100 which is practically miniature in terms of weight and size(while still falling nicely in the hand - at least with the custom thumb grip), compared to the (admittedly not very large) 5Dc. The 5D's sensor still beats the X100's, though, at least it does in my view, in practice. Especially in regards to how they work with colour(the 5Dc is just flat out better for everything except skin tones, and even then the 5D is great, it's just that the X100 is more or less the best there is in that regard...) and in regards to dynamic range. Low ISO noise is better on the 5D as well(by which I mean practically non-existant - not that it really matters, just figured I should mention it), though the X100 has an aesthetically pleasing fine(and I do mean fine - finer than 35mm Velvia 50) grain at base ISO.
Anyways, my point is the 5Dc is an excellent camera, the large majority of my photos are made with it(if you look at my flickr, you can tell which ones are made with the 5D - the EXIF info is intact)... though I've barely used it as of late, because I simply can't be arsed with bringing it everywhere with me, like I used to, because I now own the brilliant little X100. Still, on more colourful, non-cloudy days I tend to bring it with me, and if I don't, I usually end up regretting it. It's just that there aren't that many days when it's a factor at this point, unfortunately. Though yesterday was indeed one of them.