I have a G17C and a G19C. With premium defensive ammo, there is very little flash at all, as flash retardant is added to the gunpowder of most defensive loads. With cheap-o target ammo, it is a bit flashy, but I still have never had a problem being blinded or losing the sight picture. Flash is no worse than a short-barreled magnum revolver, and the revolver's barrel/cylinder gap blast carries about the same level of danger as the port blast on a "C" model Glock.
Here's a photo snipped out of a video clip of my G19C at the local indoor range, firing the Federal Champion 9mm target/range ammo from Walmart.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)