I have thick hair so I don't personally need these anyway, but I find they don't work all that well. And you can tell when someone is wearing one too... I just find they're not very useful.

Pro: If it works, it makes really full and awesome buns on the top of your head. Luxy hair has a good tutorial:

Cons: I have too much hair (past waist length), so it doesn't work on me at all. People with shorter hair tend to love it though..

Price: $2-5 each. Daiso in San fran sells them for $2, Amazon or ebay sells them for $3-7 bucks a pop. Or you can make one by using a rolled up sock (cut at one end).