Mercury and Pluto are old pals! In the days of "myth", it was Mercury's earth side, Hermes, who guided souls to the underworld after their time expired on planet Earth. Mercury was the only one both quick enough, brave enough, and detached enough to venture into the underworld and manage to come out unscathed.
Pluto likes Mercury because he's full of information and almost as tricky as 'ole Pluto himself. Pluto can use these mounds of facts to expose any sort of nastiness that needs purification and rebuilding.
Mercury in prominent relation to Pluto, be it by aspect, dispositorship, or midpoint, takes on a sharp, shrewd, tricky, probing, detective-like behavior. Mercury in this configuration has little patience for a bullshitter, hates lies and deception, and has little patience for those of slower mind. Pluto makes Mercury very sharp, in a way reminiscent of the old sprint commercials... He can hear a pin drop, and can pull that pin out of the haystack as well. A wonderful detective.. The worst enemy of those trying to fool this native. Cutting and brutal, he's great at a good debate, or worse (although maybe better in his view!), a heated argument. He can sense where it would hurt, and will hit right on that spot without flinching. Those with strong Pluto influence on Mercury are not the type to pick a verbal or intellectual battle with. They tend to cut you down in the harshest of ways, and enjoy inflicting pain verbally and/or intellectually. They sometimes (ok.. usually!) get a rise out of it. Of course, they won't always do so, but if you curse them or otherwise give them a reason... be ready! They'll say anything, regardless of social or political correctness, and desire complete freedom of verbal and intellectual action. Off limits in the mind? never! Don't be surprised if they whip out overtly sexual references, even at the dinner table. Nothing is off limits with these folks, and they make a strong distinction between thoughts and words versus actions; to them, it's not wrong to talk about or think about some of the most dark and distasteful things, even explore them in gruesome detail mentally... Even though they may consider actually doing such actions to be highly criminal or utterly distasteful, this can prompt them to actually enjoy thinking about these things.. They love to get into the mind of the criminal or otherwise forbidden subjects. The mind is very much unafraid to go where angles fear to tread.
Realize they won't display all of these attributes all the time... Those with Mercury/Pluto linkages generally receive plenty of negative feedback from those around them and learn to keep themselves in check, lest they suffer various repercussions. However, when they are irate or in good and secure company, they'll let Pluto poke through the veil. It can be quite interesting.
I have Mercury retrograde in Scorpio in partile waxing semisextile to Pluto. It is the third most exact aspect in my chart. The fourth is it's quintile to Jupiter
Most of what I wrote is from my experiences with this placement, both positive and negative. It has a some significant downsides (mainly in sensitive social or workplace situations) however I wouldn't trade it for anything. To hell with those who can't handle the intensity
edit to add: A few of you mentioned sarcasm as well. This is certainly a Mercury/Pluto thing. We love the dark but truthful and honest side of sarcasm, and wonder why all you other people think it's so bad... To us, it's a wonderful form of eloquence mixed with humor, even if you don't agree