If your nails aren't stained, the pencils do work well. You run them underneath your tips and it brightens them up.
For people who are addicted to vampy colors like Lincoln Park and Black Tie Optional, your nails end up stained all the way up. The best thing I've found for this is something I get at Sally's Beauty Supply, which is a chain store. They are little tablets that come in a packet, you put them into a glass of water and they fizz and disolve (like Alka Seltzer!). You soak your nails in there and it takes the stains out.
I don't use them often because I pretty much ALWAYS wear vamps or darks, but if I need to switch to a French for a wedding or whatevernot, I go to these. I'm sorry I can't remember the exact name.