I don't have any on my current MBP13 but had a large gray 'underworld' (electronic group) sticker on my original TiBook circa 2002. It matched the color of the titanium lid, was nearly the same height, and reflected my obsession with electronic music and DJs and clubs and all that, at the time.
I think there are a couple of rules and statements that if followed can be successful in adding some extra flair to an otherwise mass-produced piece of machinery. Stickers to me signify a little irreverence, a counterpoint to the prevailing notion that everything is perfect and must be kept perfect (and by perfect that means some other designer's notion of perfect, even if its Jony Ive).
I think the fit of the sticker, the color and size, the location and placement, and above all the right level of clarity or obscurity of the message are all things to consider. Its an incredibly easy method of customization and signifying 'ownership'. Customization can be done poorly or done well. Also, plunking a decal on what could easily cost a paycheck or two signifies a 'casual' relationship to money. This is best seen in Gumball 3000 vehicles.