From my understanding the 40+ ball was introduced to make the game more competitive by slowing down the speed a tad. I think they achieved their goal. Longer rallies do make the game more enjoyable to both play (recreational) and watch. With that being said my 40+ review will try only to consist of comparisons with other 40+ balls.
The 3 star Nittaku 40+ Premium is the cream of the crop. Unlike other balls the weight and hardness make this ball the absolute best. Other 40+ balls that you will indeed come across will be lighter, they'll be more floaty, and they'll take way more effort to produce quality spin with control in placement. In fact the other 40+ balls, with the exception of one, will nearly be impossible to control consistently. The Nittaku is truly a 3 star ball. Other balls, with the exception of one (butterfly G40), seem to be 2 star if not 1 star balls which were accidentally tagged as 3 star balls.
Serves = fantastic (DHS came close but Nittaku is still better)
Rallies = superb
Touch play = consistent (you will need to remember this ball is bigger than a 38 and thus slight more bouncy)
Spin = excellent (just watch the pro players popping balls up, they're not trying to. Lol)
Close to table play = excellent quality and consistent trajectory
Off table play = cuts through the wind (yes it's slower than the smaller balls previously used, but again the weight and hardness of the ball makes off the table play direct and you'll have confidence in your shot making ability)
Now for the bad - You can buy a 3 pack and not get one round ball (Nittaku better step it up, especially if they are going to charge roughly $3 per ball). You can buy a 12 pack and get roughly 3 to 4 round balls (maybe). And that's with being lenient and not over scrutinizing the balls chosen. Some very picky players I've seen reject some balls that I accept. But it just goes to show that there is somewhat of a tradeoff. Excellent playing properties sometimes have to come at the sacrifice of a perfectly spherical ball. But that's my only down side...oh, I almost forgot the price. $3 per ball makes you cringe when the ball is accidently stepped on. Ouch!