DS Lite Rumble Pak
I ordered mine on the day they were announced and it arrived yesterday. As you can see from the pictures, its matt black finish doesn't look too bad with an Enamel Navy DS Lite. I'd imagine it would look more out of place in a white or blue unit, and blend in perfectly with the upcoming European black unit.
It's exactly the same size as the DS Lite GBA slot protector cartridge, right down to the extended plastic ridge that makes the slot look centred even though it's a little offset to the right.
The unit feels a little heavier than the DS Rumble Pak, but in use it (unfortunately) produces much the same amount of noise, although seemingly slightly higher pitched. The rumble effect is pretty much identical -- in other words, you'll either like it or hate it, and it works best in Metroid Prime Pinball. At 1200 yen (around US$10.70 or £5.70) it's not badly priced for what you get, but don't expect anything as effective as the GameCube or PS2 controller rumble effects. (Click for full-size pictures.)