I actually use the Automix 5887 and sometimes will use Duramix 4240 I have never once had an issue with them and I also get the stuff at $15 ... they are basically the 2 on the right hand side of this picture I took.
The Automix 5887 works great for newer bumpers as it flexes just the same and you can actually sand it to be top coated if you please.
The Duramix 4240 is great for bumpers that are more rigid but still plastic. Its more for use on the rear of a bumper and then the front is to be covered with the Automix 5887.
I learned to use these at work a lot as I do a lot of plastic repairs on bumpers. Like in the winters here in Chicago cars get so cold that when you hit the bumper against anything it basically cracks all over and sometimes punctures holes. The automix can be used two ways really for filling a hole.
1st way is to cut a patch out of a scrap bumper and you glue it into place with the automix around the edge with both peices have the edges "V'd" to a 45 degree angle on each side. Then you sand both sides with 80 or 180 and apply a skim coat of the Automix 5887 on both sides and just sand the top side to your preferred finish.
2nd way is you sand both sides of the area you are filling in with the edges of the hole or opening "V'd" out about 45 degrees on both sides with 80-180 grit (your preference) then you grab a flexible plastic vinyl like formica (I believe what that stuff is) and tape it down on one side then apply the Automix 5887 to the back side and let it sit for 10min then flip it over and just fill it up to the top with the Automix 5887.
I hope this makes some sense to you...if there is something you are not sure of let me know or do a search on google I know a lot of people use the Automix 5887 product.