Yeah, the items are balanced quite fine. I usually throw items into "distribution" status, as well as fairness. By balance, I primarily mean the karts and such. Thus far, there is no overwhelmingly all powerful vehicle or combination, like in MK:Wii. Only the lightning bolt & bomb get rid of all items, as well as falling off, and that's perfectly fair. And yeah, Blue Shells & Lightning are still going to make 3 Star-ing Cups a pain.
Another thing I love about this game is that they've brought back douchebaggery! 
3-Star cups don't seem nearly as tough as they were in MKWii, as its only related to how many points you get in the end, that being said, getting to the end in first place can be tougher in the 150cc.
I will agree that Blue Shells and Lightning are given to the CPU more than I would like (At least they don't get the Lucky 7.), but I'm having more issues with the guy behind me (Which for some reason is almost always Yoshi.) getting Red Shells with almost every single item box, throwing them at me usually right before the goal or at a jump, after which I will fall, and then after that the guy in 6th place will get lightning with the guy ahead of him getting Blooper, maybe I'm just getting unlucky, but its been happening in all the levels so far, not just 150cc.
Again, maybe its just bad luck, Online will fare better hopefully, as playing against actual people is more fun.