Overall: Very solid. Definitely a hybridization of Warzone and Firefight that leans on the Warzone part of the equation, but that's to be expected.

Positives:
+Solid variety of objectives and mix of Prometheans and Covenant. That even the opening waves might be one or the other faction is a nice element, as are the random drop zones for enemies. You can go anywhere and find something to do for the search and destroy enemies, and that means the action is often less far away than it is in normal Warzone.
+REQ system works. No doubt tons more people will blow money on REQs because they get to use them in a way that doesn't feel as dangerous or as double-edged. I've definitely been burning cards I'd been hoarding unconsciously because I was always afraid of getting killed immediately when I called it in. No longer!
+Arcade fun! It's alternatively challenging, hectic, and crazy.

Areas for improvement:
-WZFF has to thread the needle on difficulty as a matchmaking mode. Remember Reach firefight? Remember how that was just Firefight Arcade constantly and was never all that fun or challenging in matchmaking? 343 clearly understands the pitfalls of classic Firefight and implemented a way that randoms and teams can have fun together. With that said, I feel like it's still a little too far on the easy side of things. Enemies can be punishing, but in my experience with full NeoGAF teams, three or four HBOers, or going solo, the seek-and-destroy modes rarely end in failure. Basically the Ultra Warden and the base defense modes are the only times you're in real danger of losing. To this end, it'd be great if we had the return of some difficulty skulls, even if they're just random and stacked in the latter rounds.
-Hopefully this is something that would be map-specific and thus not an issue, but more objectives would be great. The garage defense is tense and exciting and fun, and that's partially because of the difficulty and partially because it harkens back to classic Firefight. Maybe you need to escort a bomb to different parts of the map and hold an area for a short period. Maybe you have to do three of them in a round. Maybe in certain rounds you actually only have limited lives. Stuff like this would really add to the mode, although they might be on the edge of what is feasible for development.
-On the subject of limited lives, I think there still could be some element that dissuades death a bit more. Obviously shared and limited lives are a no-go, but maybe something like spawning bonuses that you lose if you die too often?

Overall, this is definitely a solid expansion of the Warzone experience and another reason I'll be playing Halo 5 when it comes out (hopefully with a variant on all the Warzone maps) and I'm hopeful the beta period allows them time to tune and modify things for a more polished product come (presumably) June.

What are y'all's thoughts?