One-word review: Wow.
It's also incredibly feature-rich. You have all the game modes you could possibly want, enough to keep you occupied for literally hundreds of hours. Season, playoffs, a scarily deep franchise mode...it's all here. The Road to the Show is particularly awesome: you take a player and bring him up through the minors. Reaching the majors, and then winning awards like the Cy Young, feels rewarding -- again, because of how difficult it is.
The most important part of any baseball game, of course, is its core gameplay. MLB 09 has no flaws in this regard. Pitching is never too frustrating, but it's not easy either. Unlike in other games, where your control will be perfect if you perfectly execute the given button press, this game still introduces a little irregularity to the process. It makes the game feel more realistic -- after all, in real baseball, you don't see pitchers putting it on the mark every time. Not even from Johan Santana or Tim Lincecum.
Hitting is much the same. It's a pure timing thing, not a swing-based mechanic as in MLB 2K7+. But the simpler mechanic doesn't hurt. Instead, it helps, distilling the offensive half of the game to a battle of wills between pitcher and batter. It's all about discipline here and waiting for your pitch. It really feels pretty terrible when you get behind in the count, as it should.
Overall, I've loved this game, and I'm sure any fan of baseball will love it as well.