We all have 0-lift aerodynamics in the front.
The spoiler adds zero lift to the rear as well.

At least it's functional...

 

Stillen lip spoiler looks better

 

It's personal preference, if you like how it looks then go for it. I personally don't and was curious as to why people liked it. Like I said, it totally looks like an after thought when I see one on the road.

I struggled to find a 6MT G35 when I was looking. I found two, one had the spoiler, one did not. Ironically, the dealer that had the spoiler (like a $400 option) was cheaper since they were more aggressive, but at the end of the day I just had to have the one without. I ended up paying like $500 more for a car that was $400 less. The spoiler car was in another city so by time I factored in getting there and getting the spoiler removed/filled/painted I said it's not worth it and paid the extra.

I do agree with the poster above me, the Stillen lip looks much better.