Britney's are bad for several reasons though. She can't seem to match up the colour of the extensions with the colour of her own hair. And given her hair is still pretty short (looks like she only has a few inches) and that she wears it completely straight (which probably makes it harder to disguise where her real hair ends and the artificial begins) her extensions always look really obvious.

Plus, looking at that pic from the VMAs, it looks like hers are either woven or glued in, and she has a layer of hair at her parting that's just lying over the top of the extensions, in all its 3-inch long, differently-coloured glory.
I've never had extensions of any kind so I can't comment on how good/bad it works out. But from what I understand, fusions will work out a lot better than what Britney has had over the past few months.