Incredibly awesome, yet very overhyped.
Warhammer 40,000 is EPIC. I mean, giant 8ft tall men clad in power armor battle ancient skeleton robots with lazers and small arms that launch grenades. Many characters are flat out awesome or simply to batshit insane to hate. Nearly every army is likeable (At least from a coolness standpoint) with fleshed out back stories and interesting Mythos (Except the 'Nids....they're actually kinda boring.) The forces of chaos stand out as being so utterly terrifingly badass that it's hard to describe and do it justice here.
The game itself has some balance issues, while at the same time costing out the ass to actually PLAY. GW is also sometimes very lazy about updates. The Dark Eldar went TEN YEARS before getting an update recently, where as the Space Marines get one, like, every five minutes. Otherwise, it can be quite fun if you know what you're doing.
And yet....there are problems. While the setting can be enjoyable (Especially if you take it as GW actually intended) it is filled with so much Stupid evil and random stuff happening just to be GRIMDARK that it's almost nauseating at times. The writing can also fail so badly at seeming mature and edgy that it sometimes feels like a Twelve year old took charge (The Grey Knights grinding the Sisters of Battle into holy oil? Seriously?). The setting is almost the epitome of trying too hard when you first start it (I practically HATED this game when I first started lookig it up). Finally, it's fandom can be quite irritating. Especuially if you, say, happen to be enjoying your conversation about Halo and some Neckbeard interjects to tell you about how a Space Marine could rip a SPARTAN apart, and that you're a fag for disagreeing. It's happened to me twice.
But, overall, I Still recomend the game to anyone that actually wants to try it. If you know what you're doing, it's pretty awesome, and can be a blast to play with friends.