What I got from reading Manual of the Planes

[Danchild]

If Asmodeus wants to make a soul his concubine, he'll polymorph it into an Incubus for the lulz. Dubiously better than spending eternity as a Lemure. If it plays it's cards right, maybe after a century or two it'll get a 'promotion' into an actual Devil. This is the kind of deal that a diabolist would sign up for. The majority of cultists would not be singled out for special treatment, being the pawns they are, but a devoted follower might be. A really serious diabolist would be investing heavily in his retirement plan, making contracts and reading the fine print very carefully. No doubt they would be aware of the cost of failure and lack of foresight.

Followers of actual deities may receive various benefits depending on the nature of the deity. A valued servant may become an advisor or proxy, though few are likely to be so highly regarded. Specific campaign fluff trumps general information on the nature of the planes. Worship Cyric? Your soul likely becomes a brick in the walls of his outhouse fortress. What would you expect from a paranoid, vindictive god?

As an aside, concerns about the afterlife no doubt motivate a few spellcasters to join the ranks of the undead. Escaping eternal 'reward' is probably a secondary concern or an unintended bonus for some, but for those who have really ticked of a powerful entity, it could be effective way of delaying the inevitable.

[Username17]

admitedly, since this means shoe-horning all sapient minds into nine catagories, individual planes should have inner conflicts about the finer details.

And indeed they do. As a Lawful evil person you end up somewhere in Acheron. If you happen to be a Goblinoid follower of Maglubiyet, the specific place in Acheron you end up is the giant Hobgoblin fortress world. If you are a follower of Wee Jas, you end up in her ice palace instead, which is not only a different part of Acheron, but is actually a different layer which is itself infinite from the infinite layer that Maglubiyet's fortress world floats in.

-Username17

[Ice9]

But there are evil gods of sloth who have an afterlife where you just lounge around in your own filth.

Sorry, I was being unclear there. By "lazy evil" I meant more like selfish evil, as one of the basic evil food-groups:

Devoted Evil: Devoted to a cause/deity that's obviously evil, or enjoys evil activities for their own sake, not just as a means to an end.
They would have no problem ending up in Baator or whatnot - while the road might be harder, it satisfies their desires to crush the weak or torture people.

Selfish Evil: Evil because they're willing to step on everyone (and have done so) to get what they want. But what they want is not is not inherently evil - wealth, respect, the good life.
In reality, the most common group. But the D&D afterlife (at least as described in the books) changes the equation. Because they really don't care whether they get to torture anyone or not - if the "good" afterlife involves laying in the sun and eating grapes while you listen to the heavenly chorus, then that's going to sound a lot more appealing than "spend thousands of years clawing your way up from Lemure to Barbazu or something, then you're set - unless a higher-ranking devil gets pissed at you."

Deluded Evil: They think what they're doing isn't really evil, or is for the greater good, but they're wrong. Doesn't matter whether they'd like Baator, because they don't think they're going there.

[TarkisFlux]

But there are evil gods of sloth who have an afterlife where you just lounge around in your own filth.

Sorry, I was being unclear there. By "lazy evil" I meant more like selfish evil, as one of the basic evil food-groups:

Devoted Evil: Devoted to a cause/deity that's obviously evil, or enjoys evil activities for their own sake, not just as a means to an end.
They would have no problem ending up in Baator or whatnot - while the road might be harder, it satisfies their desires to crush the weak or torture people.

Selfish Evil: Evil because they're willing to step on everyone (and have done so) to get what they want. But what they want is not is not inherently evil - wealth, respect, the good life.
In reality, the most common group. But the D&D afterlife (at least as described in the books) changes the equation. Because they really don't care whether they get to torture anyone or not - if the "good" afterlife involves laying in the sun and eating grapes while you listen to the heavenly chorus, then that's going to sound a lot more appealing than "spend thousands of years clawing your way up from Lemure to Barbazu or something, then you're set - unless a higher-ranking devil gets pissed at you."

Deluded Evil: They think what they're doing isn't really evil, or is for the greater good, but they're wrong. Doesn't matter whether they'd like Baator, because they don't think they're going there.

The whole "become a lemure or dretch and climb to the top" thing is somewhat misleading. There are more than just 2 planes down there, like the Waste and Carceri and Acheron and Pandemonium and Gehenna. There are lots of non-Baator places for the selfish evil to end up, and they're more likely to fit them better as well.

[Prak]

then that's going to sound a lot more appealing than "spend thousands of years clawing your way up from Lemure to Barbazu or something, then you're set - unless a higher-ranking devil gets pissed at you."

Actually, as lemure are mindless, you won't experience that time in any memorable way, much like your actual time in a womb and the first few months-year of your actual human life. More over, as devils progress, their personalities and minds change. Some things stay constant, such as personal tastes and quirks, and especially their grudges against rival devils that fuck them over. I would imagine that, in the case of people becoming devils, the original personality may not fully reassert itself until the devil advances up to the point where he becomes unique as an Archduke, but he will still, while advancing as a set type, likely have the same tastes and quirks he had as a person.

[K]

People tend to forget that the DnD afterlife is deeply nihilistic.

Good or evil, you end up destroyed forever. Good people end up being part of their plane or subsumed into their god. Evil people end up with one of those options, plus the exciting option of being eaten by devils. Life as a petitioner is only a stay of execution until one of those options is realized.

A rare few get a second life as angels or devils, but even that only lasts until you die again. These lucky few at least have the option of being resurrected at some point by powerful magic, but they've already gotten an afterlife and when dead stay dead.

[mean_liar]

People tend to forget that the DnD afterlife is deeply nihilistic.

Id' argue that in DnD, existence as a mortal is apparently the brief opportunity in which real change happens in the universe. On a longer scale, one's mortality is a very brief flash of moment, wherein the powers of the universe struggle to co-opt that mortality to further their cosmic goals (annihilation, tyranny, free Happy Meals for everyone, etc).

Your mortal body's death is not so much an end of your existence so much as a return to the spiritual essence you were prior to your birth.