Sixteen consoles were set up for Spartan Ops at the event we attended last night - groups of four cycled through to play an early mission for Crimson Team, and protect/rescue a Forerunner artifact. I played it through twice. Below are some thoughts - not in any particular order, and not, unfortunately, polished, because there simply isn't time if I want to get this online now. Hopefully there are useful nuggets in here.
The way it was set up last night, you had to watch the little Spartan Ops promo first, and then a CGI introduction (we saw a placeholder, with a huge sign telling us it was not representative of the final product :) ) - you couldn't skip these. Tom managed to screw up the loading by hitting the guide button; all four consoles in our group needed to be kicked back to the start screen to begin again. (Almost assuredly it will not work this way in the final product.)
We were set up with a fixed loadout - no options. (This, again, is not the way the final product will work.) There were plenty of weapons in crates, though, and you could pick up anything your enemies (or fallen comrades) dropped.
Gameplay was progressive; you start by fighing a few grunts, then a collection of grunts and jackals and elites (the mission was all about a dig that was being protected by Covenant, turned out to be Forerunner artifacts). Once you beat the first few batches of enemies, you get to open new areas, and fight new enemies. Eventually, you've killed all the Covenant - and you are confronted with Forerunner enemies (Watchers, Crawlers, eventually Knights). Before you get to the artifact the Covenant are protecting, the crates stop holding human or covenant weapons, and start holding Forerunner weapons.
Promethean enemies are pretty cool. Crawlers are fast, do relatively little damage individually, but swarm, and can be a pain in the ass if you get cornered. Watchers can catch grenades (and throw them back), protect other Prometheans, and spawn Crawlers. Knights are super-fast, deadly up close (think Hunter-strong melee attacks), and can spawn both Watchers and Crawlers. You CAN take them down with Human or Covenant weapons - but it's WAY easier with a Forerunner Scattershot.
In some ways, Spartan Ops is like firefight - you have battles against computer-based enemies, with reinforcements as you clear them. In other ways... it's way beyond firefight; you aren't holding a fixed position against attacking enemies but rather are moving forward towards your goals, all of your objectives are explained with story features, the enemy variety is large, you have goals beyond "protect this area" or "clear all enemies".
We didn't see individual medals or scoring information while we played - but I'm guessing that was a choice made by the person who set up the games. (You can turn scoring off in Reach, after all - no reason to think you wouldn't be able to in Halo 4.)
The Scattershot is a beast of a weapon. It's the same concept as a shotgun... but at range, it has a completely different function. The individual projectiles spread out and bounce - with some planning, you can hit targets around corners and behind shields. (If you treat it like a shotgun at range, it's useless; none of the projectiles hit your target. But if you treat it like a pro-pipe that fires mini-grenades... well, then it's scary. Up close, it's everything I ever wanted in a shotgun. Except once, when a Promethean Knight closed the gap just as I ran out of ammo; I couldn't reload fast enough to get a shot off, and he just swatted me across the map. DOH!)
The mission we played lasted about 5 or 6 minutes. (My second run-through was faster; I knew what needed to be cleared, and where, and where the switches to open the next areas would appear.) It's a great way to play a quick match with some friends - they're planning on releasing five of these each week, during each 'season' (5 or so weeks). There is no word on releases past Season 1 - they might continue them, they might not. (I'm guessing it depends on how popular the feature is.)
One last factoid: Spartan Ops geometry is custom-built for Spartan Ops - there's not supposed to be any overlap with standard Campaign geometry. (The entire Spartan Ops storyline takes place 6 months after the end of the regular Halo 4 campaign story.)