I think i'm a bit obsessed with milk powder dispensers!! I have LOADS!
I've got the Tommee Tippee ones, an Avent one and Mothercare's own make ones too. I found the Avent one to be the best as the powder comes out really easily. You can fit 3 x 7oz measures of formula in each compartment.
The Mothercare ones -again, you can fit 3 x 7oz measures of formula in each compartment. However, you really have to bang it on the side of the bottle to get every last bit of powder out.
Tommee Tippee ones are good... but as they're individual, they get lost at the bottom of your bag! I'm also worried about the tops coming off them. You're meant to put them inside the actual bottle with the water, but as that means having to steralise them, I don't bother.
Personally, I love milk powder dispensers as they're so convenient. I measure the formula out at night for the night/next day and it's so much quicker just pouring it in than messing about with scoops when your baby is waiting, or at night when you're up at 4am making bottles up! I use them when i'm at home and when i'm out and about. I also have to provide measured amounts of formula to Harry's nursery, so obviously have to use them then.