Primed Up

Prime me up darling!
To prime or not to prime? Why do we do it and should we?
I love most primer. When I say  most I mean I am an avid user of both face and eye shadow primer but I don't use lip primer at all really. For face primer I like to do a three part priming that I actually use to replace foundation. I first use an amazing sunscreen fluid LA Roche-Posay Anthelios 60 that is light weight, easy to apply and has a matte finish that I love. Secondly I use Benefit's The Porefessional which is a thickish balm that is supposed to minimize the look of pores and again has a matte finish which makes my skin feel so smooth and wonderful. Lastly I finish it up with Too Faced primed and pore less. I think the mixture of all three gives me a great all day coverage that let's me skip foundation which I always have problems with.
Eyeshadow primer does pretty much the same for your eye lids but tends to be thicker and you can buy it in tons of formulas from almost every brand. I'm a big fan of Benefit's stay don't stray and urban decay's eyeshadow primer potion. I think they help your lids from getting oily and making your eyeshadow wear off or fade away. Whenever I use primer my eyeshadow looks just as good when I take it off as it did when I put it on. I also run primer along my bottom lash line to help my liner stay on and in place. 
Lastly I have tried lip primer but I'm not a fan I actually just prefer some Chapstick about 30 minutes before I put on gloss or lipstick to make my lips soft and moisturized. I feel that this works just as well and cost a whole lot less.
How do u feel about primer? Does anyone else swear by it like I do?