I recently gave Iron Saga a whirl as well, had been debating to bring it up here.
The PvP (which thankfully seems to be not a major part of it) definitely has a P2W component what with the progression curve, but there seem to be pretty reliable 'free' strategies available. Balance is pretty mixed, which means that SSS rank mech you get might be quite useless.
The plot is a basically a slurry of political melodrama, anime, aspirations of Cowboy Bebop, sprinkled with the occasional stink of male chauvinism. Could be better, could be a lot worse. Also the translation is sometimes terrible.
My favorite part is probably the sheer variety of units, and that you can use pretty much any of them (to varying degrees of success). Want to use a crummy mass-produced tank on your team? Sure. Some of the lower-ranked units punch way above their weight. There are mechas (obviously the focus), tanks, and aerial units, each with their own basic class aspects (e.g. tanks can reload weapons quickly, aircraft can auto-dodge attacks, mecha get fancy techniques but can be knocked over) and individual unit perks. As Folly notes, the variety is quite vast but the easily-available reviews do help.
Tip: In menus that feature silly eye-candy art, click the button in the lower-left corner to change the portrait to one of your pilots. Thus you can tailor the level of your cheesecake.