I had a Kate Spade Matthews Street Camplin Satchel Bow Bag and a Newbury Lane, and personally I've found MK to be much higher quality. The leather on the Matthews Street Camplin felt thin and like plastic. The Newbury Lane was in the same price range as the MK Sutton at the time on EBay, but the leather on the Sutton is more substantial and feels better and thicker than the Newbury Lane. To me, the Newbury Lane also feels a bit like plastic or some synthetic material. Maybe higher end items in Kate Spade are of better quality, but MK delivered on quality for the same price as Kate Spade's Newbury Lane.
I've had my saffiano MKs for a few years now, and they all look as good as new. They've held their structure and because of their hardiness, they haven't shown the effects of daily use. They've been accidentally scraped on some surfaces (not very rough surfaces though), but the saffiano wasn't affected. The bottom corners haven't lost their colour and aren't scraped. This definitely 100% makes MK worth it in my opinion, because I've paid for quality and I received it, and the bags are holding up very well with time. I don't tend to buy trendy designs (studs, grommets), so I'm sure I won't have bags that look dated or that don't suit my age as I get older. Who knows though, some people are saying the classic Selma is beginning to look dated, which I completely disagree with!
I have the MK pebbled leather jet set messengers, but the leather dried out even though I condition them multiple times a year. They feel like dry skin, which is so unpleasant! Does anyone know how I can fix that? I don't buy pebbled leather from MK anymore after I experienced this with two jet set messengers. Their Sloan looks better quality for soft leather, but is way more expensive.