I think that he was probably a major asset, at least until Generation VI. His music is consistently great, and he was the director for a lot of games.
>Gold and Silver - Sub-Director
>Crystal - Director
>Ruby and Sapphire - Director
>FireRed and LeafGreen - Director
>Diamond and Pearl - Director
>Black and White - Director
>X and Y - Director
I think that he really pulled Gold and Silver together after he was put on the team in 1998. The beta ROM is fascinating but I think we ended up getting a better product in the end. Tajiri had a really cool vision, but I'm not sure how competent he was as a game director. Red and Green took over 5 years of development and is a buggy mess, but the core concept shines through enough that you can look past the flaws. The beta GS ROM feels a little off. It's not fair to judge an incomplete game, but Game Informer's interview with Masuda paints the picture that development was a total mess when he came on board. I think that with Masuda guiding the way in a sub-director role, the team was able to make a better, more cohesive game, and I think that's probably why Tajiri backed off and left the series to him.
While it's easy to say that Emerald, Platinum, and B2W2 were all better than RS, DP, and BW, I think it's only because Masuda had made those strong foundations. Like no matter what, USUM was not going to be a great game because SM's foundations weren't there. And I hear a lot of people saying that XY could have been fixed with Z, and I think that's evidence that there were some strong foundations, but I also think that they could have used another year of development time to further those foundations. Prior Generations had around 4 years of work put into their flagship games, but XY only had 3 years and also required moving the series to 3D. The team size was increased, but I think it needed more time. Lately, he feels like the series producer much in the way Eiji Aonuma does for Zelda.