It's hard- I think- to separate commander's personality and sometimes the uniform they wore from the genuine work they did.
Longstreet was human- he could be obstinate, stubborn, perhaps back-stabbing (towards Bragg) and self-important and maybe a little slow, maybe he didn't handle Reconstruction 100% right, and maybe he got bitter towards the end- he could also be reliable, loyal, hard-hitting, intelligent, a good friend and husband, someone who understood the importance of reconciliation after the war, someone who had a lot of reasons to be bitter and someone who Lee trusted greatly. The former traits don't necessarily cancel out the later.