Choice Scarf - Locks you into one type of move and increases your speed by 50%
Choice Specs- Locks you into one type of move and increases your Sp.Atk by 50%
Choice Band- Locks you into one type of move and increases your Atk by 50%
50% is a massive boost. The fact that you are locked into one type of attack is annoying but a quick switch will fix that right up.
Tbh, you can't say you've played competitively until you've used choice items
Hope I Helped :D