The DLC was marketed as progressing the timeline to 3057, but the last news blurb is from 3052; Certain pilot academies will open after that, but clantech remains segregated to Clan missions and never filters into the Great House militaries gameplay-wise, outside of rare market deals and random drops from arena prestige ranks.
The storyline is an enjoyably tough romp through clan space from the first days of the invasion, with an abrupt 'Halo 2'-style cliffhanger ending. An ironic twist of fate makes you pair up more with friendly lances throughout this DLC than when you're attached directly to Great House militaries in previous ones.
The vast majority of the soundtrack absolutely rips, with 'The War Machine' being one of the best tracks in the game; Legion of the Brave sounds like a demo take they left in, but it's still better than 'Master of War'.
The new mechs and weapons add some welcome build variety for your own stable, but the implementation is somewhat rough around the edges; Damaged arms that float, broken walk cycles, clantech weapons that look visually indistinct from their inner-sphere counterparts, weapon sound effects that sound oddly quiet on certain mechs, weapons that don't work like they should in certain omni slots, etc. The upside is that there's so much they added that despite the few rough edges, you'll have hours and hours of new toys that actually work to play with.
The new Biomes are effectively recolors, but they still add a welcome amount of visual variety while running just as well as the old ones.
The new pilot progression system adds some welcome depth to your mechwarriors, but it's still easier to 'trade out' a worse mechjock for a better one on the market than it is to spend the tens of millions of C-bills necessary to train up a bad one; You'll spend more on tuition than on some assault mechs.
TL;DR - If you like the base game this DLC is a must-buy, but if you don't then it still won't change your mind.