When I bought Rimmel's Wake Me Up foundation recently, I also picked up the Wake Me Up concealer. If you read my review on the foundation (here) you'd know I didn't have the best experience with it. The concealer is another story, though.
I have this concealer in the shade Soft Beige. The coverage is pretty light for a concealer so it's not the best for blemishes but it's a really nice under eye concealer. It is marketed as a full coverage concealer but I wouldn't agree with that. It doesn't feel heavy or look cakey, it doesn't crease and it had a sort of cooling effect, too!
I dot a little onto my under-eye area and blend it on with a MAC 224 brush. I was using my Real Techniques Domed Crease brush but I found it was just buffing the concealer away completely. I think a fluffier brush is an essential when using this concealer. If I think one layer of concealer isn't enough, I can easily add another.
I have pretty bad dark circles; in the photo below I've just used the Wake Me Up concealer.
The staying power is pretty good for a concealer which feels so light on my skin. It definitely hangs around for a lot longer than the foundation does!
All in all, this is a really nice product and for me, it's much better than the foundation. I picked mine up from Boots for £5.99 (link)