I play as either one of my created factions (I have BattleTech Clan and Inner-Sphere factions and a faction that is like Morgan only useful) or one of the following: Svensgaard, Yang, Lal, Santiago, Aki Zeta-5, Miriam or the Cultist (whose name I can't think of). I prefer the fast, early, overwhelming attack styles that Spartan, Hive, Believing and to a certian extent Cultists can pull off, I play my best games as the Pirates, and Aki, but Lal and Deirdre I do well with aswell. Lal is the only one I play timidly with, Cult and Gaian wind worm armies can be devestating, going around with supior weapons as Aki, Yang or Santiago work well, and overwhelming the enemy with large numbers of inferior troops is always fun as Miriam and sometimes Yang. I don't play as the aliens, the Data Angles, or the free drones, and i prefer Cult of Gaians, and I don't play as the University now that Aki is available, although the free nodes was awsome for the early game+alien artifacts.
I agree that Miriam, and to some extent Santiago seem to push their vision on the player more then some of the others, althought I think Yang is guility of the same thing. The University seems to hold the others in contempt, and I think it is hard to decern what Lal's agenda is, and to what lengths the Gaians will go. I for one love the Social Engineering aspect, the choices of general government, priority, economy and future ideal seem very interesting, and I always talk with my brother and my friends on what other things could be added, whether more options to the existing categories, or adding another section, like foriegn policy(IE: Isolationist, Aggressive, Cooperative.) I do not Engineer on my ideals, I do what I think is best for that faction, at that particular time and situation, and I for one love making "vassals" of the leser factions, leaving them with a handful of bad cities and just feeding my beast with the added commerce from the pact.