Just installed mine on my 2010 Touring and can say its a big improvement. I have the DDM slim ballast 35w, 4500k bulbs. Although I'm very satisfied, the 55w is probably that much better. Like many others said, do not go over 4500K color for the best performance.
I bought the 55w kit for my wife's CX-9...still have not installed it. Bought it back in Dec of 09. Whatever, hopefully I will have time this weekend. In terms of brightness, I have the 35w 6000k kit on my Accord, which is a perfect blend of color and brightness (just a hint of blue, but for the most part, white). So, based on their recommendation on their website (below), I got the 55w 8000k kit for my wife's CX-9.
55w kits tend to wash out some of the color of the HID bulb due to the light output( for example, a 55w 8000k will look similar in color to a 35w 6000k kit).