TR is the best unique item I've found yet, so I'm pretty happy with it. I had Grunsmanns Bow too, but salvaged it....noob mistake lol.
I've only used it on Threatening so far, but it easily clears rooms with either the Archer or Hunter.
Archer - Archer's Lance from as far away as possible. Follow this up with an Explosive Shot, and watch the fireworks begin

Hunter - Leaping Shot from close range, follow up with Rolling Draw, Stealth, Explosive Shot auto-Crit from Stealth...fireworks

Or, agro a group of mobs, and run around a corner (Chateau is great for this). Kill them one at a time as they come around the corner. The explosion from the first kill will prime the next guy to be easily killed with 1 shot, which primes the next guy, etc, etc.
The obvious "key" to getting the most from TR is to make sure you're getting killing blows. If you're in a group where everyone is competing for/steeling kills, the bow will not shine, as it's somewhat low in the DPS department.
If you can get in a group that will let you use the bow to its full potential, it can be deadly.
Or you can PuG, be that d-bag Archer/Hunter that runs ahead, pulls an entire room just so they can get their gun off, and then let's everyone else clean up the mess.........the thing is though, if you play it right, with TR, you can clear an entire room by yourself, and be that d-bag Archer/Hunter that steals all the kills

If I ever come across Grunsmanns Bow again, in definitely going to keep it to experiment with that %17 Attack.