There can be bad parts, but the ending should be a good part.
For example, at the end of WoL, the Zerg had been all but destoyed and Raynor had succeded, but billions had died and Tychus was killed.
Raynor and Kerrigan: yes. If not, we totally wasted all our time in WoL and HotS.
If it weren't for the fact that all of our missions in the games so far were in an effort to protect and/or save Raynor and Kerrigan, or kill the person who is specifically trying to kill Raynor and Kerrigan, then I wouldn't care about them, but that is what happened.