Altra Instinct 1: My first experience with the Altra Instinct was from watching my wife run in them. I was trying more minimalist shoes intially, but switched to the Altras for my 2nd Marathon. I only ran in 1 pair of Instinct 1s before switching to the 1.5s.
Altra Instinct 1.5: I ran several marathons (races 2,3,4) in Altra Instinct 1.5s. I really liked the soles and zero drop platform, but I was never really happy with the construction of the uppers in the Altra Instinct 1.0 or 1.5 - they seemed to me to be prone to tearing and wear a lot faster than other shoes. So when I saw that Merrell was reintroducing its Bare Access in a completely remodeled shoe, I switched back to them.
Altra Instinct 2 - so far, my favorite iteration of this shoe
Altra Instinct 3 - nice looking shoe, but with some changes over the Instinct 2s
The Altra Instinct 3 (top) has a more curved sole than the Instinct 2 (bottom). Consequently, it runs small in comparison to the Instinct 2. I still need to try the Instinct 3 in proper size (for me, an 11) and I may find that I like the toe-off in those shoes.
The sole layout is changed very little between the Instinct 2 and Instinct 3