The career mode is definitely what this game needed for me. The campaign is good for a first time playtrough, but the Career mode really has that "Pull yourself up by your bootstraps" feel. It's great, and you don't even need to get the DLC to play it! On the other end, you can continue your campaign save and play the endgame flashpoint missions, which I haven't tried.
Done my first flashpoint mission, which was unintentionally hilarious: They make a big deal about how great the Hatchetman's melee ability is, but the first combat casualty of the day was that 'mech's hatchet, turning the poor 'mech into a regular man (It still has an AC/10, but okay). It made an already very tough mission even tougher: That hatchetman replaced a heavily damaged Vindicator, and was supposed to be the saving grace for me not being able to repair my mechs between missions... Missions which I completed by the skin of my teeth, to boot.
I highly recommend y'all give it a go. If you do try the career mode, remember that getting salvage rights on assassination missions is a really good way to get yourself some mechs beyond your current weight class (which will be important, as you'll start with lights!)