Yes, everyone's been to Baator, and the whole party loves it and its denizens. They have several contacts over there, including the Archduke of the 1st and 2nd layer, Bel (I'll get there), who on terms "as friendly as possible" with them after they helped in a huge ritual that allowed a powerful Avatar of him to be summoned to the material plane.
There are reasons for this - its because the party is basically cursed with having to kill demons and venture into the abyss to face demon lords or plot bad things will happen. Therefore, they managed to struck a careful deal with the devils after a lot of political maneuvering, and of course they ended up on disadvantage, but they were acceptable terms. The lesser evil, if anything.
Now, to go into more detail about the current situation of Baator in the setting, there are currently 10 layers of hell instead of 9, ever since Asmodeus was dethroned, the fight having ripped open a new layer where the current ruler, Eligos, resides. Bel took a great part in helping the Eligos defeat Asmodeus, and he was given Dispater's former domain after he too fell in the uprising.
As for the events leading to Asmodeus downfall, those are as complicated as the abyss, but suffice to say that it involved (but was not limited to): Vecna doing stuff, Asmodeus' mortal cult being wiped out, Levistus getting freed from the curse, Zargon the Returner returning, ancient baatorans, the Ruby rod being destroyed, the primeval pact being torn asunder, angry devils, angry angels, angry inevitables, and a lot of good old backstabbing.
Other Archdukes that are no more and were replaced in the uprising include: Glasya, Mephistoteles, Belial & Fierna.
After the major power shift in hell, things are more or less stable, and the upper planes would say the changes were for the better, even if not ideal. That concept of balance is also enforced by the god of neutrality, who got pretty strong in the meanwhile.