I have tried a couple of primers, and I have another one on the way to try soon! However the MAC Prep and Prime is definitely my favourite, the slight illumination provided is really handy when you aren't a pro at highlighting.
I often wear the primer with just a light layer of powder foundation over the top when I am in a rush, or not wanting to wear much make up due to either running out of time to put it on or general laziness. It makes the product stay beautifully and is great for evening out skin tone even though it isn't a true 'colour correcting' primer.
It is quite expensive, however 1-1.5 pumps provides ample coverage and if you know you are going to use it regularly then you can pick it up in the occasional sale.
This was my first ever primer I bought and I haven't found one as good since. I haven't tried another high end primer though as they are just too expensive for me, so have to stick with drugstore now.
I use 1 pump for my entire face. It glides on like a dream, moisturising and smoothing. My skin feels like velvet after application and makes my foundation apply flawlessly.
It helps my foundation last longer which is one of the main reasons I buy a primer (I do not like to powder my face at all).
I only have two complaints. 1. The price. It is sooo expensive I have never bought it again even though I loved it! 2. Although I like the look of the sleek packaging, the bottle is not see through so you cant tell how much you have left.