Not much to a feeler gage really. If they are nicely ground, they won't rust as easily. If they are nicely heat treated, they will flex without bending permanently. The edges shouldnt have burrs, like they were stamped out. I have a set of the older Starretts. The markings are engraved or Chem milled so they don't wear off. Beautiful old serifed type. Should be able to find them on eBay for $10.
Bent ones are handy as are the long way ones. Snap on made a set for valves with hard handles.