So some reviews are in,
http://www.giantbomb.com/borderlands...?review_id=247
And the word around the internets is that it sucks.
And I have to agree, particularly with everything Jeff says in the review above. My first character is level 42 (I got distracted by some other games), and after I finished up zombie island I was like, ok, the new DLC is a horde mode, I can probably finish up those last 8 levels there.
Wrong.
Each of the different horde areas has 5 rounds, composed of (I think) 5 waves. The first round is easy. After that the game starts doing modifications to you and the enemies every wave, and each wave also has a theme. So for example, you might get the "shotguns do increased damage, everything else does less" mod and have the "bad ass" wave.
By the time you get part way through the second round, things are getting tough, even on the first playthrough. Did I mention that these rounds take forever to complete? For each of the horde modes, you're looking at a significant time investment.
Now, this wouldn't be a big deal if you got good loot, xp, and/or weapon proficiency along the way. But you don't. Like, zip, zero, nada. I played for about 2 hours on one of the hordes and was not able to beat it, and I have nothing to show for it, not even a single experience point.
The frustration here is compounded by the fact that you could get unlucky with mod and wave themes. For example, one wave I got the "you have no shields" mod and the "guns" wave theme, which resulted in me being in the center of about 20 Crimson Lance dudes who each one dropped a turret, and me having no shields. And if you lose a Wave, you have to start all over. And again, you will have -nothing- to show for it.
In conclusion, if you haven't spent money on this DLC yet, do not do it.