After putting a bunch of mods and miles on my 2015, was one of the first production runs, I now have 3000 miles on the 2019 and thought I review the major differences I have experienced.
1. Quality of assembly, large improvement. On the 15 at least a third of the body screws was stripped, cross-threaded or cracked plastic. After disassembly on about half of the body panel I have only ran across one screw cross threaded. I was very excited about this as was expecting much worst.
2. Shock valving better, not great. While still a little harsh over sharp cracks and bumps, it does have a better ride than the 15. Some of this made be related to number 3 below.
3. Frame stiffness, large improvement. The old chassis would flex like crazy during rapid transitions and twice after hard mountain riding my Bullet Top frame would no longer line up with the mounting holes. The 19 stays flatter and has better control during hard corner transitions.
4. Engine tuning. The 19 has more mid-range torque, dont know if this is the change from air box to the CAI design, but two others have also noticed the difference.
5. Gear box whine much less. I am counting this one as just a production fluke as their is a lot of sound variation from one slingshot to another.
Overall I like the 19 better in large part because of the reduction in frame flex which gives me better corner control.