The S40 is a great bike, with one major design problem that Suzuki has been aware of since the very beginning, but never fixed it. I don't know why, it would be such an easy thing to fix. It is the cam chain tensioner. There is a do it yourself fix for it that seems to work very well, but if you ignore it, the cam chain will eventually break or jump teeth.
The S40/LS650/Savage was created on a small budget back in the mid '80s as a mid displacement cruiser that could be sold cheap. When the cruiser craze hit in the mid '90s, Suzuki brought it back, with an additional gear, but still using the defective cam chain tensioner. It's just as easy to fix as the KLR650 balancer chain tensioner, or doohickey, which Kawasaki has also ignored for decades.
But even with that small problem, the S40 has proven to be WAY more durable than any Royal Enfield. You can cruise all day, day after day, at highway speeds, without any fear of it breaking down. Even with the RE complete redesign, they still can't seem to get it right. They do seem to be fairly reliable if you do not go over 50 mph, but my Vino and Zuma 125s will do that.
2002 Vulcan 750, 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
2012 Zuma 125, 1980 Puch moped