There's a recent discussion of furminators here: furminator chat. I use it on my larger Corgi who has a really thick coat, and I love it! I usually use it a few times a week, and each time I just focus on one area -- his furry turkey legs one day, his chest another -- because he's got so much coat! My little girl has a thin coat, so I don't use it on her at all, I think it would be painful or uncomfortable. I have the medium size, and it's good for us.
I love love love the furminator. I bought the small one so that I could furminate under her legs and chin. I furminate Maggie once or twice weekly along with daily brushing. Right now she is shedding her winter coat so we have fur everywhere. I highly recommend it.
Can't live without the furminator if you have a corgi - they work better than any other type of brush when Charlie, the live shedding machine is blowing his coat. I don't know what size it is but it was worth the money.