I had a 2013 and just bought a 2017. I liked the looks of the original 13 but the 17 has grown on me. I liked the 13 round fog lights better. The interior is a bit more refined on the 17. I do like the daytime led headlights on the 17. They look sharp to me. I can't say they drive any different stock from what I can tell.
Had a 14. Just got a 17 a few months ago. They are a bit different. The 17 seems to feel a little more refined in that it shifts a little smoother the shocks and springs seem a little better sorted out and the ST1 seats look a little nicer. I did like the look of the 14's front end better.
The character of the car hasn't changed though. Still a blast to drive.
As a 2013 ST3 owner, I've thought about it as well. I love the updated front end. I too loved the round "halo" fog lights but the way they've narrowed and sharpened the front end makes it look much more aggressive. With that being said, I absolutely HATE what they did with the rear end. I'm personally not a fan of the center exhaust placement but I can deal with it on my 2013. I would feel compelled to do SOMETHING if I had a 2015+ and changing it to something better costs a pretty penny right now. When it comes to the interior, the main additions from the 2013 ST3 to the 2017 ST3 is some carbon fiber trim pieces, SYNC 3, and the heated steering wheel. While I do wish I had the heated steering wheel, it's not a big deal for me personally.
For me, the minor interior changes and the bittersweet exterior changes aren't worth going through the whole car payment thing again. I'd say if you had a 2013 ST1 and were considering a 2017 ST3, then I can see how it'd be worth it. To each their own.