I have used primers on and off for a couple of years, never been very convinced as to their usefulness.
Read about the Loreal Professionals one a couple of weeks back in Sunday Times Style and it said it was the best primer available.
It is excellent. You put it on and it makes your skin feel ultra smooth, and covers blemishes to the extent it looks like you have got foundation on. PLUS my foundation now stays on all day, instead of falling off shiny areas, and settling into dry patches on my cheeks. It is probably the best £12 I have ever spent on make up.
(Don't work for Loreal btw)