I'll start a new topic for us to discuss the PEI (Polyetherimide aka Ultem) print bed surface.
Firstly, this seems to have originated from discovering that SpiderBot is offering a bed with this surface for their delta machine in Europe although there are earlier discussions on various 3D Printing forums.
In the case of SpiderBot, they have laminated a thin ("close to 1mm") sheet of PEI to aluminum to use as a print surface. The material is supposed to be great for PLA and ABS.
I ordered a 12" x 12" sheet of .03" from Amazon and it came in today. This is .76mm so technically "close to 1mm".
The material is flexible at this thickness and looks like light colored honey - very transparent. I clipped it to my borosilicate glass just to see what it would look like:

As you can see, it is shiny and transparent. Clipping this thin sheet on the perimeter will not remove all of the waves. Even thicker sheets are likely to be flexible enough to lift from the center of the large Rostock build surface. Ideally, the thin sheet should be adhered to a flat substrate.
Just for grins, I decided to position a print close to the edge where the material is clipped reasonably well and see what happens. The print - PLA - is in progress now with a 60°C bed. I cleaned the surface with isopropyl alcohol, adjusted my X, Y and Z lengths by subtracting .76mm from them and fired off the print. It was amazing to watch how well PLA sticks to this stuff. I'm printing one of my "open weave" fly fishing reel parts that is very challenging to stick to a bed. The PEI is sticking nicely. I'll post updates when the print is complete.
To remove the part, you are supposed to run the plate under cold water. If this is the case, it would be good to have 2 of these so I can print and remove parts without interruption. I'm going to try to find some thin, flat aluminum to glue the PEI to. Short term I might just stick it to one side of my borosilicate to get going quickly.