These little gizmos are fabulous for swatching colours and I have spent most of today painting mine. So far I have done all my Nails Inc,China Glaze, OPI, A England and Color Club and have filled 9. Each wheel has 18 nails. I have yet to swatch Leighton Denny, Boots, and various other miscellaneous brands.
The wheels can be bought on eBay for a few quid (I paid £1.99 for a pack of 10) and i think could be my new best buy, especially for trying colour combos, comparing similar colours and trying special effects. A second pack has already been ordered.
Here is a picture of the ones I have done so far and include Tuesdays TSV from Nails Inc.