I bought MAC Prep & Prime Highlighter in Light Boost (£18) on a bit of a whim - I thought it looked pretty and liked the idea of a highlighting pen. MAC say that this highlighter adds radiance just where you want it. It comes in a 3.6ml pen type applicator with an end that twists, releasing the product into the pre-built in brush. Personally, I use this after my foundation, around my eye area to brighten and conceal.
Prep & Primer Highlighter is not a full coverage concealer, it reminds me of a heavier Touche Éclat. It really does help to brighten under my eyes, like the Touche Éclat, but also it adds some amount of coverage to my dark circles. I've also tried using it around the face as a highlighter, but don't really feel that this works too well - the colour is very yellow toned and I don't really feel it 'highlights' any areas of my face. Under the eyes however, it works well to conceal and brighten.
MAC Prep & Prime stays put for most of the day (I hardly ever touch up my makeup as I usually don't have time) and I haven't found it to crease; probably because the formula is quite light and of a fluid consistency. I am not sure whether I would repurchase MAC Light Boost, as £18 for 3.6ml of product is very expensive and I think there are cheaper options out there. The product works and helps to cover and brighten my dark circles, but it just hasn't wowed me. For brightening under the eyes, I much prefer my Smashbox Under Eye Brightener.