I just finished the campaign yesterday. I was very excited about it when I first saw it. I would love to say it's a good game, and it has alot of good things (which youtube and steam reviews are posting no small portion of). I certainly won't say it was a bad game, but without repeating what other people have already said, I have some problems with it because I felt largely disappointed with this game. Let me begin with its virtues, while trying not to repeat what everyone talks about in depth.
Pros
1) A present plot of sorts taking place during one of the major campaigns in 40k history (I didn't mind the voice acting too much)
2) Nearly all inclusive by way of units (Forge World and Games Workshop both)
3) Almost all Imperial Guard units are Steel Legion themed (some crews are still Cadians)
4) Very classic and well structured game mechanics
5) This game can be played without ANY space marine support (which I largely found inferior to fielding more Imperial Guard hardware). Finally a game that doesn't cater to GW's "space marines are what it's all about" theme.
6) The weaponry works fairly true to canon and tabletop standards (a Punisher gatling cannon is good against infantry, but does nothing to vehicles). Basically, if it'd work on the tabletop, it'll work here (and vice versa).
Cons
1) Beyond the dialogues between missions, the plot is absent to the gameplay. I would have liked some cutscenes (at least a closing one). Instead, the campaign ended and it just sent me back to the main menu. Additionally, they added some elements that seemed rather unnecessary (such as the inquisitor showing up and trying to "recruit" you).
2) The forces you can assemble lack a sense of proportion to the realism of the 41st Millenium. In the final mission, I fielded approximately 10 Reaver titans, 8 Basilisks, 9 Steel Legion infantry units, plus the "command group" of Yarrick, 2 chapter masters, the drill sergeant, and the detachment of Ultramarines. For easily the later half (if not 2/3 starting in part 2), the titans and basilisks took up the VAST bulk of the fighting with infantry moving up only to capture objectives. I didn't feel like titans were rare assets, or limited in number available to the war waging across an entire planet.
3) By way of the Factions (Imperium and Orks), the armies/clans within them are almost completely left out. They mention extensively about how you're part of the Armageddon Steel Legion, but they make no mentions to the Ash Waste Militia, Hive Militia, or even Jungle Fighter regiments that Armageddon maintains in its PDF (the best they get is Hive Gang Militia units, which are hardly regimental strength). Stormtrooper companies and every Ork clan (like Goffs, Evil Sunz, Snakebites, etc) are completely left out. Variances between the Space Marine chapters are minor at best.
4) While a hex based game brings back ALOT of good memories for us long termed gamers, for a 2014 game, I expected better graphics, unit models to face all directions (that was mentioned in another review). Aircraft function like aircraft, not another ground unit who's only unique feature is they can move over impassible terrain like rivers and mountains. A more prominent role for the VIP's of the campaign (Dante, Yarrick, Ghazghkull, etc).
In short, Warhammer 40k: Armageddon is a nice game, excellent in how it even breaks from the Games Workshop ultimate sanction for space marines and their typical themes of Cadian uniformed guardsmen praising the Angels of Death for saving them. They incorporated almost everything a 40k player (particularly an Imperial Guard tank player) could want for their army. The game mechanics can be said to be reasonably good. However, this game has no feel of being new. I would have expected to see this game as it stands back in 1995 (give or take), not 2014 (almost 20 years later). It also still needs some work to get completely in tune with the 40k lore (like Imperial Guard regiments, Ork clans, etc). I was hoping for a GREAT game, but found a decent one instead. Even with it's 20 year retro and my minor grievances regarding 40k lore to game practicalities, I'd still suggest people have a go at it.