It's very realistic, it's only faults being that the electrical/hydraulic system is not completely realistic (everything works, but, for example, you can operate the control surfaces with no hydraulic pressure) and a few bugs. The bugs aren't too bad, though: they're just minor things like a couple false warnings that occur when loading saved flights (and by the way, you need to load saved flights twice in order to load the saved settings).

I know I made it sound really bad with that, but the truth is, I don't really notice those faults. I will say, however, that both are older products, so they might be just a step lower than what you're used to. If you don't own, say, the 777 yet, I'd recommend you buy that (especially because v1.0 is going to be released soon, and you'll have to pay full price once that happens), but if you really like the 757 and 767, they certainly wouldn't be a waste of money.