See maokai pretty rarely, he looks like a fun decent support champ, is he any good or is there a reason people dont play him?

He's tough. And incredibly blue reliant. But he's damn good if played well.

People don't play him, because doing good depends on having good teammates. He can CC the hell out of the other team, while ganking from the jungle, but if his teammates don't capitalize on that CC he's useless. His ult prevents a ton of damage, but only if your teammates are smart enough to stand inside of it.

TLDR: If you have sucky teammates you will suck with Maokai.

Don't do support Mao. I prefer going jungle, as with his e (WEEEEEEE) you have a pretty good clear time, and your ganks are good. He is severly underplayed, but very good if you know what you are doing.
Also.....
he has 1 spell that supports (ulti)
his e wards, but you are better off using it to clear minions, or harass champs

I've used him in the support role as a counter to Blitzcrank if Alistar is unavailable. He has great brush control, and provides alot of initiation and teamfight potential that many supports don't have. That said, he has very little sustain(just his passive for himself and nothing for his carry), and no good escapes, and his only harass is his brush control, and has a habit of chasing down and last hitting minions that your carry needs.

He has some advantages, but in almost every situation, another support would be preferable.

Full Tank jungle Maokai all day. Carry so hard with it if you get good.