I have a 50 spines/ice scrapper and it's ok, but I don't love it. That's mostly the spines part.
I skipped Ripper because I can't stand the animation, but to me it's the logical place to put it for a few reasons. It's got one of the highest probabilities to proc of any Spines power AND it will hit up to 5 targets in a 90 degree arc with a 7 foot radius. I don't know about you, but I'm generally going to get at least three enemies in that arc as long as there are at least three enemies left. If your base chance to proc is 50%, then your base chance to NOT proc is also 50%, and if you hit three enemies then your base chance of NOT proccing on any of them is 1-(0.5*0.5*0.5) = 87.5%. Downside: If you are only fighting a single hard target, your chances obviously drop; Ripper might not even be in your rotation for a single target; putting the proc in Ripper means that Critical Strikes won't be active for Ripper.
Also, Ripper has a long animation time before effect. That means that if Critical Strikes activates during Ripper, there is not a ton of dead time before you can make the next attack. With 2.574 seconds left out of the 3.25 seconds window of Critical Strikes, you can get off Throw Spines and Lunge (possibly Quills!) as opposed to just a single attack.
That said, I currently have it in Spine Burst and with up to ten targets it activates almost all the time, but I will never get off more than one attack afterwards with Critical Strikes active. Except Barb Swipe and something else, but I also don't use Barb Swipe.