Urethane, when used at the right time will help a bowler control the backend that many reactive balls provide. They do, on today's conditions, have their limitations.

They respond much slower to friction.  That can be good or bad. They need to be wiped off religiously since the oil will sit on the cover longer, which may cause the ball to not read the lane as easily as a reactive covers. They usually will not allow the bowler to open up angles very much due to the above reasons.

If you want control, and would be willing to sacrifice some "miss room" on the lane then urethane balls can be a great addition to an arsenal.  Not usually your everyday choice, but can save your bacon when the lanes get ugly. 

By the way, the Karma Urethane is at least as good as anything out there.  You should not have to pay a premium price for urethane either.  Good luck.

My personal favorite is the Grenade by Roto Grip, but I feel the new Blue Hammer is the most versatile in that it will hold a polish and not completely lose reaction. It is generally about midrange in strength. Hype urethane is probably the strongest, but I have not tried the new U2 classic.
Hope this helps,
Joe

Technically not urethane as that process is not allowed in the US anymore
"Urethane like" would be better

Just drilled the U2 classic and its slightly longer than the blue hammer or natural
predictable and has a spot in my bag always

Where i see the diffrence between the U2 and the naturals is the carry %.
The carry % of the u2 is almost as great as the reactive balls where i can leave flat 10 all day with the natural and natural pearl..

Charlest - I had the Natural, went to the blue hammer(which I still have)

Now drilled the U2, 60X5.5x40
The main reason I went away from the natural and the BH is no matter how they are drilled, they seem to roll very early and then roll out

the U2 seems to have a bit of slip in the cover and the carry is good
Im on old wood lanes THS and it gets goofy sometimes

I did take the cover to 4000 dull and after 5-6 shots with the cover getting some oil in it, it was more of the shape Im looking for

I would be so interested in another urethane ball. Thinking about grabbing a Desperado LE. Blue Hammer looks awesome as well. I will have to look at the U2...

The blue hammer has a 4.75" pin-pap
this one is longer

these are not your urethane balls of the old days
I believe the process to make an actual urethane ball is not legal in the US today

the blue hammer just rolls earlier than i like and then pretty much goes straight
I have had some success with it on a burnt shot at a tourn that had wood lanes
had to loft it and go pretty hard up the track

this U2 is better, but not a giant difference, it still has the early roll characteristic
if there is some heads, this will be great, no heads like im seeing, it still rolls early and has less carry because its spent when it gets there

I had the blue hammer and many different surfaces
the U2 says its 2000 on the website, looks like 4000, rolled somewhere in between

took it to 4000 on my spinner with the pad marks parallel to the track
it did make a difference in my opinion

might do 2000 or even 1000 with some polish for my next experiment