Maokai doesn't suck; as someone who plays him almost as much as Rammus in the jungle, I can tell.
While his sustain in the jungle is pretty low, his ganks are extremely effective, and, fun fact, Sapling Bomb has one of the highest base damages of all abilities. Also, his clear time isn't all that bad, and I think people tend to underestimate his ult; it can take a VERY nice chuck off the enemy team's health if you play defensive.
Maokai is also decent as a support or a top lane.
His passive isn't much to talk about in mid-late game, yes, but I think people just don't like Maokai because he's just hard to play, and needs some runes/masteries to get him going in the jungle.
As for Renekon, he's a MONSTER. I bought him on the NA server, and if you play your cards right, your abilities can just shred the enemy's health to PIECES. I almost consider him OP after playing about 5 games with him.