If you can get to late game with him; he's an incredibly nasty beast. As mentioned, though, you have to be prepared for a glass cannon start; your team has to understand you're not going to be a mad early ganking beast or a superb tank then.

If you get fed though...

They will feel terror. His w gets ridiculous in late game especially with spirit visage.

hes not overpowered, he is a carry jungler and is not easy to have a lot of success with in s3. you really need to know what you are doing to succeed with him and it takes some practice. that being said, he is a really good pick in the right hands

As with most champs he is fine. If you play him well he will be good and if you don't he won't. He isn't a free win like release xin or diana not long ago or khazix sorta now but he also isn't karma.