I find Guts to be a sympathetic character, if only because we see a lot of his childhood and can see how that and his lifestyle (mercenary) have shaped him. He never knew kindness of any kind until he was forcibly 'recruited' to the Band of the Hawk. We got to see his growing relationships with Caska and Griffith. Even though he was a stupid bastard for leaving, it just shows he's human and makes mistakes.
I think that's it. He's not perfect. He doesn't really know how to be nice. But he does try his best with Caska and Griffith.
As far as 'abandoning' Caska - I see it more as deciding to go after the cause of everything bad that happened (Griffith, and the manga has already established that these two are fairly obsessed with each other) before the curse can catch up to him.
He has all the odds stacked against him, but he's still fighting. I think the readers can't help but respect that, while also feeling sympathetic due to how things fell apart.