Okay, I've never read this pairing before, maybe frightened off by tripperfunster's art. :) You really characterized Filch well, dirty and resentful and ornery. I really liked how he helped Snape as a student, and then at Sally's, when Snape kept repeating, 'D'you know it's my birthday?"
They haven't seemed to change much in their older years, and that's what I like most about this. The thought of Snape watching/wanking to Filch in the shower, Filch still calling Snape 'a good lad.' It all rings so right and true.