I've played all of them a fair amount, except in ME3 SP, where I've only played Engy and Sentinel all the way through. I'll agree that Engineer doesn't seem as thematic as some of the others, however I think Sentinel can work fairly well for Shepard (at least Fem Shep). As far as ME3 MP, there are kits I enjoy for every class... hopefully ME4 MP will be as good.