This was a question brought up in a recent discussion. We often think that, in general, batters tend to hit a pitcher better the more they see him.
Among teams Mo's thrown at least 30 innings against, the Angels have fared the best, putting up a 3.43 ERA, followed by the Mets at 3.18, then the Orioles (3.12) and Red Sox (2.89). Against the other two division teams, he's hardly had any problems - 1.91 ERA vs. Toronto, 1.63 vs. Tampa.
But I wanted to look at individual hitters. Mariano Rivera has been in the majors since 1995. He's thrown almost 1300 innings (including the post-season). The following is a list of the players who have faced Rivera the most in his 16 seasons (including the playoffs) -
Above is just the top 10, but looking at the whole list, you'll notice a lot of the hitters played for three teams: Boston, Baltimore and Texas. Two division rivals and a team the Yankees played several times in the post-season.
There are only two players (among the top 10) that Rivera has pitched better against than his average OPS Against of .552: Johnny Damon and Trot Nixon. That means that 80% of the hitters on the above list have hit Rivera better than average. Of course, this could be expected considering the talent on the list.
Only one of the top 10 has "enjoyed" facing Mo (i.e. fared better against him than their career OPS), Rafael Palmeiro, who somehow OPS'ed 1.026 against the future Hall of Famer. For reference, Albert Pujols' career OPS is only a tad higher (1.050).
So it does appear that hitters who see Rivera more hit him better... but it ain't quite that simple. The players he's faced the most have generally been above-average to great hitters. That makes sense - the better a hitter is, the longer he'll play the game.
That leaves us with another question: do the batters hit him well because they've seen him a lot, or because they're just good hitters? (Or some combination of the two?)