Like your favorite pair of jeans...
...this baby never goes outta style. I have been using MAC Studio Fix Plus Foundation powder since I was 16 years old. While I'm not a huge fan of MAC's foundations in general (they are too reactive to most ppls skin) this powder foundation is one that I never leave behind.
At any one given time, I always own two of these: One to carry in my purse, and one to keep at my vanity at homebase. Currently I'm wearing N4 and apply with a MAC Kabuki brush (SO much softer than Bare Escentuals brushes, btw). It evens out my skin tone, lasts all day, and doesn't clog my pores.
On good "face" days, I use this by itself. On "okay" face days, I put it over a tinted moisturizer and on "ick" face days set my foundation with it. It applies beautifuly in all situations without being too powdery and never cakey. It is the perfect not-too-sheer/not-too-heavy alternative to things like Estee Lauder's Double Wear powder (I'm color: Ecru in that).
Oooo and don't forget that you can Back 2 Mac these babies. Save 'em up and turn six in for a free Lipstick! My picks: Craving and Cosmo