I don't feel Rexxar is very good. Misha is too easy to kill and if she dies Rexxar basically can't do anything. He's a warrior role but he isn't really super beefy either so he isn't really a tank. And if you do have a proper tank on the team, he's kind of a waste of space. I think Blizzard needs to change some things about him.
I think he's pretty decent. The bear is actually fairly tanky, and can get down right ridiculous if you talent for it. The only time it dies easily I feel is due to mis-micro, as you should be able to pretty much heal and call him back out of almost anything. Unless of course the enemy team spends a lot of resources on him and that's still a really good outcome... it's like when the enemy is blowing cooldowns on a murky.
He's definitely not a main tank though. It's really hard to find the right comp for him but he fits more into a bruiser or specialist role. My favorite thing about him is that he's got really great talent options. He's got great talents for damage, wave/merc clear, sustain, and CC. I think he works best paired up with long range damage dealers, dive partners, and CC combos.
I really like him on infernal shrines, and he's probably also really good on battlefield and cursed hollow. A lot of tight hallways that you can just care free send the bear into for vision and disruption. It provides a lot of opportunities to get free damage and strong initiations on the enemy team. For example, on infernal shrines during the objective you can sic the bear on an enemy hero and get free damage and vision of them until they decide to do something about it, and then you call it back, heal it up, and do it again.
You put that on an enemy backliner and they've got a real problem. They either need to waste cooldowns on what's essentially a summoned murky, or reveal their position and take free damage. You combine that with anything that can follow up and as soon as they waste cooldowns on the bear they're dead. You're essentially forcing a fight, you just need to be sure that your team is ready and willing to follow up.