I played around with the level 17 owlbear, in part because of all the complaints about how lousy it is supposed to be. I had mixed results, and while I think the beast needs some work, I do think it has value in certain circumstances.
It does pretty decent damage.. around 50 a swing with crits as high as 150. Its trip attack is also pretty good, likely due to its strength. You can get extra damage out of it by hotkeying its cleave and greate cleave feats, but of course this is more work.
When I took it in the Vale, I discovered that it appears to have a bug of some sort -- on occasion it just goes to sleep and stops doing anything until I call it. It wouldn't even respond to being attacked.. and no, it wasn't on passive mode.
It is not terribly useful in a party situation in a real quest. I summoned it into a Coal Chamber run and it did a bit of damage, though not a lot. It died a couple of times by standing in traps, typical boneheaded hire behavior. Overall, it was not too impressive, compared to five human players.
But I also summoned it in a short-manned Elite Beyond the Rift and it racked up quite a few kills there.
Where I found it really useful was in soloing stuff. I summon it and the panther and a healer hireling and I have my own private army. I had great fun with this in the Eveningstar cavern challenges; their presence made a big difference due both to drawing aggro and adding DPS, and the humanoid hireling ensured that they didn't die.
Overall, I think it's useful, when used appropriately. I would like to see its defense beefed up a bit and the bug where it goes to sleep fixed.