Mussina has been my favorite pitcher since he came in the league. It pains me to say but he belongs in the Hall of Very Good and no more.
He's going to finish with some great numbers by the time he's through but not Hall worthy numbers IMO.
I somewhat discount the 20-win thing. In 1995 or 1996 he left the final game of the season up 2-1 after 8 innings. Armando Benitiz then gave up a game-tying homer and he finished at 19. That's not even the most painful event in his pursuit of 20 wins. In 1994 he was 16-5 and was a LOCK for 20. Heck, he had a shot at 25. Then the strike.
His playoff performances have been middling. Overlooked, however, is 1997 when he struck out 25 Cleveland batters in 15 innings. But snakebit similar to the 20-games thing, he lost both games because of lack of run support. His ERA in that series was 0.60.
Further overlooked was his series saving performance out of the 'pen (his first relief appearance ever) in Game 7 of the 2003 ALCS against Boston. The Yanks were down 5-1 with no outs and runners on the corners in the fourth or fifth inning. Mussina strikes out the first guy and then gets the speedy Damon to ground into a doubleplay. The rest, as they say, is history, Mussina's performance again being overshadowed, this time by Aaron Boone's series clinching homer.
A couple personal comments: I started collecting autographs of athletes through the mail in the early 90s and continued for about 5 or 6 years. Mussina was one of the first to respond to one of my requests.
I was so thrilled to get his autograph, I immediately joined his fan club. I immediately got a standard thank you letter for joining and a couple other items. Then there was a span of 6 months that I got nothing. For kicks, I wrote to him to his hometown and mentioned this. Within 10 days, I got 2 different bundles of merchandise from the fan club. Coincidence? I don't think so.