So far I got Mag to 30 and love her. Shield polarize is all sorts of useless but her other three powers are nice! I bought Frost and Vauban the other day. Frost's first two powers SUCK! I hope snow globe and avalanche is freaken worth it. I absolutely love Vanban's little sticky zappy spheres of death. Then there is Excalibur prime.... Got some cool powers, but holy shit with all the polarity slots that I have NOTHING for. So for me right now: Mag - 30 Excalibur Prime - 19 Vauban - 6ish Frost - 4ish Considering Rhino or Ember next. I have all the blue prints to build Banshee, just need the mats / cash.
Frost and Vauban are a god send on high level defense, bastille or that snow globe will stop anything from getting near that damn pod. I have ember and I love it, running with ember and flamethrower is all sorts of satisfying. Mods for excalibur prime are all power/offense, need to grab those range and constitution mods for them. Rhino is built like a brick shit house, I mean the thing never dies and has insta kill everything buttons. I leveled my mag to 30 there last week and by god is it fun, pull crush pull crush pull crush all day long. I kinda want to build vauban next, undecided though, may grab rhino just to be like wargrunt and solo every boss if I want to. (Also, shield polarize can be useful if really highly leveled and with a trinity around, but it is the least used of my mags powers). Ooh, and banshee is a fun one to play with also, got her at 30 just 'cause. Maybe try a loki? 'twas my first frame and the most lulziest of all of them, wait until we get together and you find me being chased around by a million dis armed moas.
I have only found one of those Y'ed arrow polarized mods for frames so far. Gives 5% (well 15% now XD ) duration to powers. Slapped that bitch on Vauban for long lasting zappy turrets. I also get my Dethcube and throwing knife things today !