"What do you think of "express" sights for handguns, specifically the Browning Hi Power or CZ-75?"
Not, much! I don't like them at all on any handgun and see them being useful perhaps only for a person having some vision problems. The Hi Power, CZ-75, 1911, etc., are all capable of being shot quickly and accurately with standard configuration sights that offer some vertical "guidance" as to where the POA actually is. The very shallow "V" on the express sights does not do this and the "ball" front sight is not as tall as several of the fixed sights in terms of being "high visibility." At least this is how I see from using these sights. That said, I've seen some precise shooting done with these type sights at both close and long range, but then "regular" sights can do the same thing. I've seen some might fast shooting done using these sights but the same can be said for the older traditional sights as well.
Originally used for "dangerous game rifles" intended for use on large animals at very close range, the express setup has merit. The sight radius between the rear and front sights on a rifle is much larger than that of a pistol and the difference between the thin blade or small-width front rifle sight is more striking than with the usual 1/8" wide front pistol sight width.
The times I tried the express sights on a service size automatic, speed was not increased, but accuracy usually decreased for me. As I found no advantage in performance, I opted to stay with that which worked just as well at speed, while still retaining the ability to make a more precise shot if needed. I find the express pistol sights to be more difficult in this regard. While I've heard other shooters say the same thing, honesty compels me to say that some folks insist that the express type sights enhance their ability to shoot accurately at speed.
I've seen the argument that they offer a less cluttered sight picture such that you can see what the opponent's doing with his hands, etc. This doesn't make too much sense to me for the following reasons. With the pistol at a high ready and looking over the top of the gun, if the decision's made to shoot, the gun's raised and the focus is on the front sight at that point. The rear sight and the target will be slightly blurred.
Others may like them and if so, use 'em. Each of us should do what works best for our own unique abilities and preferences. For now, I'm sticking with the older type, more traditional sights on my handguns.
My suggestion is to do what you believe works best for you.