Comment by namelosw
Well, as a northern guy, I do find myself able to understand Mandarin even from Yunnan easily without prior learning. The harder ones for me, like the Hefei dialect, are because the pronunciation is very different, not the tone. Nanjing dialect, on the otherhand, is also from the same Jianghuai Mandarin group as Hefei, which is perfect intelligentable for me.
Even for non-Mandarin/Guanhua, such as the Shanxi dialect, I can understand them because the pronunciation is much closer to mine, just the tones are completely novel.