I get a discount at the Tropical Smoothie Cafe by my house/gym.

I usually get an Acai Berry Blast with Whey added in. It contains Acai, pomegranate, blueberries, bananas, and strawberries. Before the whey added in, the nutritional values are:

431 calories
0 carbs
13 calories from fat
1.6g fat
5.3g fiber
9g sodium

The whey is 22g worth of protein when added in. Antioxidant wise it's a good bet I assume.

But more importantly, how beneficial is it to drink this DURING my workout (24 oz)? Can it replace the after work-out shake since I'm drinking it DURING the session instead of water?

Lastly, I've been mixing my whey shake mix with milk alot of the time. Milk is casein "based" or slow digesting. Does that kinda defeat the purpose of drinking a shake immediately after a session for quick protein absorption if mixed with milk rather than say fruit juice?

I drink a water based shake during my workout and then one after. I am no scientist so I can't answer your other questions but I can say I am seeing good results these days.

There's no problem mixing with milk. And actually, a casein-whey blend has been shown to be better than a pure whey or pure casein blend.

Personally I dont like mixing with Milk as find it heavy on my stomach.
Aside from all the nutritional facts, when it comes to protein I have learnt that one of the most important things is that you like how it tastes. That seems like half the battle. The only issue is that some of the ones that taste really nice are not as good as some that dont taste so good.
Recently I tried a natural protein and although it did not taste awesome I seemed to be able to digest it really well and it had good effects.

no way that can that have 0 carbs
I wouldnt put too much value in their nutrition facts

I also dont think mixing with milk will do any harm. Rather it will only benefit you