The piece of metal is very flat in printing with the alligator clamps I'm using.  If it is left unclamped, the heated metal bows such that the edges stick up.  However this is not an issue since the clamps hold it down.  When it cools, the metal is flat again.  There is no permanent warping, and my parts always come out flat.

The attached file is a Winterfell print at 0.1mm (from Thingiverse) on the metal bed.  I am very satisfied with the performance, especially the reduced "thermal penalty" from swapping plates, and the general durability.

Though the Aluminum one may have a better time for pre-heat.  May have to check it out as it wouldn't be too expensive a start up.

The part pretty much always pops off the glass if you let it cool first.

I totally agree: I never pry anything off.  I just wait until it cools, and it pops right off

I wonder how a copper plate would work.

I use a Cricut spatula to remove prints.  It works sweet on Kapton.

I think someone tried that already here once.

I use a Cricut spatula to remove prints.  It works sweet on Kapton.

The plate takes about one minute to cool enough for easy part removal.

Josh, I think the next thing I will try is a thicker plate.  I'll see if there's any significant warping.

I have been using a glass bed / hairspray for the last week or so.  It has great adhesion, but recently I had some issues with gouging and cracking (gouging meaning chunks ripped out of the glass surface when I pried a part off)..

I think your prying to hard if your pulling chunks of glass out.  How thick was the glass?

Haha and yeah the glass definitely had some great bonding.  Here's a picture of the glass gouging:
I was using a screw driver to pry under the part, and just holding the plate with my other hand.

I can't see how you'd take chunks out of the glass, how were you holding it and removing the prints?  That's some extreme bonding right there.