I was a little wary of this product after some of the mixed reviews it got. Sure it's a slightly different silver, and it's plastic rather than brushed aluminium, but it works just fine.
It sits alongside your keyboard and the connector works great, so it looks like one long keyboard. Not only is it good to have a numeric keypad back, but the Delete ket function is great to (I really missed this), and Home and End keys too!
All in all, I would recommend this to anyone not wanting to buy the wired Mac keyboard who needs a numeric keypad.