I like them, generally. The idea of hordes of monsters comes immediately to mind for a game with DOOM's plot, and these maps pretty much realize that, even beyond what they may offer in playability.
I find some of them to be somewhat frustrating or too hard, but that doesn't stop me from appreciating the feats of some players on those maps, such as surviving a tough slaughter map in a first demo attempt, without or with few deaths.
Lots of the negativity about slaughter maps is tied to their general difficulty, and I don't approve that. Yes, a map with 300 imps in a room may be boring, but most slaughterfests are much more hectic than that. This just attempts to use the worst levels of the type to mark the definition, ignoring all the complex and difficult maps being praised by slaughterfest fans. It's deceptive and hides a cowardly critique against that general difficulty. Saying you're bored by killing 1000 monsters when you're given equipment to smash them relatively quickly if you're good is a bad argument, and killing less monsters with a minor arsenal isn't necessarily quicker or more fun.
Something many explain some conclusions, though, as the better slaughter maps tend to be the harder ones, meaning that weaker players can only play the ones that tend to rely on easier, even stupid, strategy. But that doesn't stop these people from watching others play and seeing the levels they are generalizing about don't fit the definition drawn from their more limited experience.
^ Edit: this fourth paragraph was inserted after Phml's post below, although I had not seen his post yet, so we started talking about the same issue at about the same time.
I think it just refers to the amount of monsters, after that it's a matter of appreciation. For some "slaughter map" or "slaughterfest" is a kind of insult and for others it's an ideal, and then that ideal can vary. This simple definition also leaves room for a good or bad slaughter map. A level with "unique and difficult challenges" tends to be a good map, as that is a positive appreciation, and all genres have low points. That can also be done with few monsters, anyway, so it's not a quality specific to "slaughter" scenarios.
I'm unsure where many people draw the line and start calling things 'slaughtermaps'.