When this "trend" (apologies to everyone who hates this word) first reared its head last fall I was admittedly opposed to the whole notion of rocking sweatpants as part of a classic, understated kit. Looking back now it seems as though what actually bothered me was the notion of wearing tailored slacks in jersey cotton. I still think that particular incarnation of the pant looks kind of silly, but I am fully endorsing your average, run of the mill sweat pants.
One of the cornerstones of finding your style is being comfortable in your sartorial skin and sweatpants is a great way to achieve this..."comfortable" is as much a state of mind as it is how you actually feel in your clothes. I honestly believe you can achieve both as long as your aren't all gym teacher-ed out, although the guy in the last photo looks dope. Pair the bottoms with a nice oxford/sweater combo and some classic sneaks and you're golden. It's not difficult as long as you keep the sweat pants a classic shade of gray and not ill-fitting (I wonder if at any point in history anyone has taken a pair of sweatpants to the tailor). Keep your eyes peeled for a pair like the Save Khaki or J. Crew joints below that are probably on sale this time of year.
[Picture courtesy of Mister Crew.]
[Pictured courtesy of Backyard Bill.]