I didn't have a taco handy and I was too lazy to go get one so I just stuck a finger covered in Diablo sauce in my mouth.
It singed with heat on contact and got me coughing a little but I didn't need a drink or anything. There's a nice spiced flavor to it as well as a bit of fruitiness. I ended up eating it with some chips and kind of wished I had a taco... It didn't feel quite as spicy as the Sriracha Taco I had recently but that may have been due to the quantities involved.
If you like to douse your food in various hot sauces, Taco Bell's Diablo sauce will probably just be a slight tingle for you. If your idea of spicy is jalapenos, it might make you cry. If you're somewhere in between and are looking for some heat, it might be just what you're looking for.
While the sauce will only be available for a limited time, Taco Bell will be looking into customer feedback before deciding if it will be made permanent.