Yes, he has been good, but only over a stretch of 8 or 9 starts has he been that dominant. There are many pitchers in the game that can run off a string of starts like that (Miller second half last year, Mussina at the beginning of this year come to mind.). He has not had to pitch a full half season up in the majors and has not had to face any quality lineup a second time through, which the pitchers that pitched the entire season have had to do.
Here are the numbers:
Nomo: 9-8 (due to crappy run support), 2.71 ERA, 1.11 WHIP, 108 Ks
Batista: 6-3, 2.80 ERA, 1.25 WHIP, 74 Ks
Willis: 8-1, 2.13 ERA, 1.16 WHIP, 68 Ks
Bottom line is both Nomo and Batista have better numbers, especially considering they have had to go through lineups second and third times. If Brown or someone else can't pitch, both are far more deserving than Willis.