I don't have one, but want to get one. Does that thing on the bak looks a lot better? Any block on the back view?

IMHO these things look like a Dodge Caravan from the back w/o the spoiler. Looks aside, it makes a tremendous difference in the amount of dirt accumulated on the back window. Before I got my spoiler, I would have a layer of dust accumulated on the back window/bumper just driving home the few miles from the car wash. Very annoying! BTW- I got mine off Ebay for a hundred bucks and change, painted to match. Install was a piece of cake and I believe it is well outlined by another member elsewhere on this forum.

Hmmm.... The spoiler doesn't really matter to me right now, but if it keeps the dirt and dust off my rear window and bumper better then I'm game! Blackberry Crush shows just the tiniest bit of dust...

The spoiler does slightly increase drag, but not to much due to it's aerodynamically neutral design.  So, it somewhat affects negatively mileage, but not too much.
Mostly, it is a matter of looks and if you like it or not.
Personally, it's neutral to me, more a matter of your intent in style and whether or not it fits.