so you are asking to what extent they are similar? i thought compatible meant something else, but i stand corrected.
anyways, most forerunners are quite similar, there will be generation gaps though e.g. 245 to 255 is a huge leap, while instinct 2 to 3 hardly any.
hard to compare 955 to i2 though - hardware is different, screen, sensors, software, it's all different.
what similar is - actual functionalities (platform), but you could compare this way most of garmins...
biggest difference, apart of hardware, would be maps, some high end things like stamina, for sure sport profiles, battery management, some glances and widgets, satiq, really quite different watch. I see it first hand having 255 (little difference to 955) and I2.
