Well, my guardian I used Kira all the way from 15-50. She is an incredibly awesome companion, and I thoroughly enjoyed her story, and just generally having her around. And I continue to use her when I do dailies, just because shes in 45+ purples, whereas Doc is in greens, with a bunch of slots empty even. I leveled as Vigilance, and the double dps combo was quite fun, even if not efficient.
My Shadow actually used Iresso
from the time I got him all the way to the end,simply because I wanted to pursue the romance, and being Infiltration, having a tank companion was great. I have since gone back to Theran for dailies and stuff though.
My assassin used Andronikos until I got Tallos, and I have never gone back. My Marauder was the same, never leaving the healer once I got them, outside of quick dabbling with new companions as I get them. As I get into my later 50s, I've seen most of the leveling experience multiple times, and now I just try to get to 50 asap, hence using the more efficient companions.
Since these days my 50s rarely do anything outside of pvp, I tend to have zero comapnions out anymore, as I find it unnecessary clutter while standing on the fleet.