I don't know if that was me who mentioned it, but if not, I second that recommendation. Perhaps not the most protein content (22g per scoop) as compared to others, but it's the best tasting protein I've had (with water or milk). It should be noted that it has a slightly higher level of saturated fat (2g, 10% daily rec.) and cholesterol (60 mg, 20% daily rec.)
Also, a BlenderBall/gravy mixer/actual blender is suggested as it makes any protein shake thicker and more like a milkshake.
In regards to the flavors of Syntha-6, I have had Cookies & Cream (not heavy, has chunks of "cookies" in it) and Chocolate Peanut Butter (heavier, more dessert like) and both are fantastic. I've heard that Strawberry is supreme, if you enjoy Strawberry flavor.
If anyone is interested in Chocolate Peanut Butter, you can do a Subscribe & Save option (and can cancel any time) on Amazon for a 5lb tub for $37 shipped:
Syntha-6 Chocolate Peanut Butter 5 LB.
It is completely acceptable to do HIIT on P90X cardio days. Those people, including myself, that have done a P90X/Insanity hybrid routine can attest to this; however, with the increased intensity in your cardio exercises, you become more susceptible to injury so be careful. I tweaked my knee during HIIT, so I've gone back to a straight P90X2 routine. Just find what works for you!
In regards to the consuming protein pre/post workout, it just depends on your preference. Some swear by pre-workout and others, post.
Personally, I have it after my workout. It's important to note that most protein shakes do not contain the optimal carb/protein ratio (4:1) for a post-workout drink. With that said, I usually have a cup of Chocolate Silk Milk or Chocolate Milk, followed by a protein shake.
During the workout, I find it's good to have motivation to work through it in order to be "rewarded" and the shake that I drink (Chocolate Peanut Butter Syntha-6) is quite heavy, so if I took it before, I'd be quite sluggish. Again, it's completely up to you and every body is different, so just try things out and find what works best for you!