Law of Ueki starts out rough but improves by leaps and bounds with each arc. By the time it ends, it's a classic. The author has a knack for eccentric fights and landing jokes.
The first few volumes are awfully clumsy. The weaknesses come to a head when it revises Ueki's powerset and introduces some plot concepts that got very close to making me give up the series. I was smart to push past the growing pains, because if you can get to the Dogra Mansion arc and you still want to see more powers, you are in for a treat. From that point forward the quality of the series takes a dramatic ramp up and keeps improving. The pace grips all the way to the end, and the series ends exactly where it feels comfortable, without rushing or dragging. There's no filler, and the powers just keep getting weirder and weirder. You will see some characters with truly surprising and scary abilities.
At its heart, this is a manga about weird superpowers, kooky character designs, and the power of friendship. The recipe for Law of Ueki? Dragon Ball setpieces + goofy reject powers from JJBA and Hunter X Hunter + the art style of early One Piece + the couldn't-do-it-without-you-guys attitude of Kongou Bancho.
Some of the jokes and ideas are truly ingenious. Mori, the powerless sidekick, is a wonderful character who lands gag after gag, and just because she's the comic relief, don't expect her to throw fights.
She gets a power early on, but she's not told what it is, and there's an incredible long-burning lead-up as she gets closer to figuring it out. It takes like 13 of the 16 volumes to be revealed, and when she's trapped in a life-or-death fight and finally understands it... it reduces her to angry tears, because it's the stupidest power in the entire series. And yet, at the same time, it might just be the strongest. Because from that point on she sure as hell doesn't lose.
The main caveat is that this series runs on pure undiluted FRIENDSHIP.
The author is too nice to kill anybody, and the villains are ALWAYS surprised that Ueki's devotion to his friends lets him survive anything, even though they've seen like 40 people in their position lose for the exact same reason. Villains are rarely evil; most are just frightened, lonely people who were corrupted by misfortune and cruelty... which can get exhausting. Personally, I love the designs and powers enough that I was willing to troop it out.
The author admirably avoids a Goku scenario by having every one of Ueki's friends contribute all the way up to the end. In fact, of all the "power of friendship" mangas I've read, this one has the purest heart and the strongest dedication to the concept, as it never lets Ueki shine without letting his friends shine alongside him. Every character is allowed to grow alongside him and remain useful, even the cowardly and weak ones.
If you're willing to stick it out until it finds its feet, you'll have a lot of fun.