"Cross Game" is one of Adachi's best works.
The first volume of manga is one of the very best I've read among all the manga I've read. Adachi does an amazing job of building up the relationship between Koh and Wakaba, showing little details that make you sure they are right for each other, even at a young age. He does an even better job of showing just how jealous Aoba is of all the time Wakaba spends with Koh. Then tragedy strikes.
Adachi's portrayal of all the different reactions from family, friends, and acquaintances is spot-on accurate.
The interplay between the characters often leaves you guessing about Adachi's final plans for the series, but that's a good thing. As I've read almost everything by Adachi, I was able to guess correctly about how the main characters would end the series, but Adachi did throw a few curves in the middle of the series which left me guessing. The ending was not contrived, however, and it left me feeling satisfied (in stark contrast to how I felt when I finished the "H2" manga).
The artwork shows Adachi's further progression in style beyond his previous works. While remnants of his overall feel from works such as "Nine" and "Touch" still remain, his style has matured quite a lot since then. I still enjoy his soft lines and effective portrayal of emotions and action (including his often-included comedic action).
All of the main characters were very believable, and most of the supporting characters were good as well. The only character who didn't seem to progress or grow much was Senda. While he matured a little as a player, his personality showed little change or maturing from junior high through high school. This is the main reason for the less-than-perfect rating for character.
Overall, I really enjoyed this series, and I've added it to my list of all-time favorites. If you enjoy romantic comedies (with some baseball thrown in for good measure), I highly recommend this series. Even if you don't generally go for sports manga, try this one out and you may very well be surprised and grow to like it. The baseball is used only as a setting for the overarching romance (even though you can tell Adachi really loves his baseball).
Go. Read it now. I think you'll like it.
(edit) Some idiot complained about me revealing something which happens in the FIRST EPISODE, how the main characters deal with that, and me talking about how the main characters have a goal to get to Koushien. Hopefully the review still makes sense, since it now likely doesn't flow as well as it used to. Chalk it up to over-sensitive pansies who think any plot detail is a spoiler.