But seriously. They're doing an 'Advent Calendar' for December, with each day offering up a new digital download for purchase. The downloads are in the $4-$5 dollar range, just right for an impulse purchase. December 3rd's was of note for me, because it contained a new Tau Empire formation, one that just so happened to match exactly the contents of a large box set GW just recently put up for sale.
The Firebase Support Cadre has to contain an XV104 Riptide and two squads of three XV88 Broadsides, though the drones are optional. It costs no additional points, and curiously, can be taken in addition to Allied forces, without requiring the player to run a core Tau force; you could, if you wanted (and could somehow find the points for it all), run a Marine army, with Inquisitor warlord, Allied with Imperial Guard, backed up by a Firebase Support Cadre. 40K is getting weird, guys.
Anyway. The benefit for buying this formation is that every model in it gets Tank Hunter and Bane of Angels, a flowery way of saying Preferred Enemy: Space Marines.
There's been a lot of arm-flailing about this, but while it is an improvement (more, useful USRs for free is always better), it's not a spectacular one. The XV88s get an only marginal benefit from Tank Hunter, because they were already relying heavily on sheer weight of fire to strip hull points, and three XV88's firing at anything they can realistically hurt (AV12 or weaker) aren't likely to leave a vehicle alive after a firing pass. It actually serves to make the old railgun-toting Broadside seem more attractive, as Tank Hunter removes the uncertainty of just straight up rolling a 1 to penetrate with your single-shot S8 AP1 gun; given that the heavy rail rifle is more likely to cause an explosion, rather than just strip hull points, and can get through AV13 (though not easily), GW might actually have been on to something when they decided to depict the XV88s on the box as being half high yield missile pod, half heavy rail rifle.
Or, y'know, they might have just wanted to show the two build options for the model kit. Whichever.
The Riptide, however, does get a pretty decent benefit, which is silly because if there's one thing Riptides don't need, it's to be stronger. Tank Hunter on its basic three-shot ion accelerator is decent for dealing with light vehicles like Rhinos and Raiders, but where it really shines is in conjunction with a fully Nova-charged accelerator shot; ordinance means that you get to roll two d6 and pick the highest to penetrate with your S9 AP2 shot, and Tank Hunter means, heck, just re-roll those two dice if you need to, to really make sure. If you can catch multiple vehicles under that blast you have a pretty good chance of destroying them both, which means parking lots have to look out even more, now. Bane of Angels is also useful, as being able to re-roll To Hit against Space Marines essentially gives the Gets Hot large blast of the weapon free twin-linking, and the re-roll To Wound means you can basically be certain you will absolutely annihilate whatever Marine unit you put it overtop of.
To quote Andre the Giant, there will be no survivors!
The Firebase Support Cadre's abilities are balanced by its high cost (minimum 570, with absolutely no upgrades) and requirement to be purchased as a bloc.... if you're taking these units for the first time. For Tau players, or players with Tau Allies, who have been used to running a Riptide and a unit or two of XV88s, the introduction of this formation is going to be almost entirely profit. You pay nothing more, and get two useful (but no, not game breaking) abilities, for units that were already worth taking, anyway.
There's some debate as to whether or not you can purchase the FSC as a Farsight Enclaves unit, rather than a core Tau Empire unit, with access to the Earth Caste Pilot Array and the six-point bonding tax for the XV88s. If you can, it will make the Riptide moderately more capable at the already-stellar job it's doing, but if you can't, well, it's hardly going to be a serious setback to anyone looking to put this formation on the table.
The rules for the Firebase Support Cadre can be found on the Black Library site, here, and the 2013 Advent Calendar can be found here. It alternates randomly between 40K and WFB, so if you're interested, check back often!