I had a 3M bra on my Lexus IS350 for about 4-5 years, then it started to cloud a little bit where it was too visible so I took it off. I will say, I never got any rock chips, etc with it (obviously) and have had the bra off for just over a year and have MANY already. Id say if you get a quality kit (such as 3M), its worth it.

Can this stuff be used on the bottom side of a PAINTED car or is it worth using it in this area?

I know there is a seam that you can see but is there any difference in the shine between the paint and the part that has clear bra installed?

If you think that area is prone to chipping,than it's worth it.You can use it anywhere.

If the paint has been wetsanded and buffed to a slick finish, you willl see slight difference,the surface has slight texture , it blends well with factory paint with orangepeel, not as well with slicked out paint

Agreed, mine has orange peel.