Curry is a solid PG defender. He's not all-NBA defense caliber but he's certainly a net positive on that end.
As others mentioned, his help defense is really good (that's his elite skill on that end) and penchant for getting in those passing lanes to get steals. He doesn't get a ton of steals off bad gambles as other players do.
His frame is his weakness so while he has solid footwork, he can be shot over or bullied more often than others. But, again, he's a net positive on defense.
The Curry-Klay-Iggy-Durant-Green lineup is by far and away the most dynamic defensive lineup in modern NBA history. I've never seen anything like it.
For fuck's sake, in the first 2 games the Cavs...a historically great offense, was decimated. Last night the Cavs hung some points up, but the truth is it took amazing offense from Lebron and Kyrie to pull that off. A lot of that was not generated from great looks but rather simply those two making tough shots. They're amazing offensive weapons and that will happen from time to time but it's not sustainable.
For all the talk about the Warriors offense, obviously warranted, it's their defense that is truly superb. Your only chance is to have five offensive weapons out there at all times. Unfortunately, no team has 5 guys like that that play defense, too. So you're boned.