I've owned and used Swarovski, Zeiss and Kahles, both in the 30mm European models and the 1" American market models. I also own Leupold, Bausch & Lomb (Bushnell's Elite 4200) and .22lr scopes.
I cannot say anything bad about the quality, clarity, or function of the European scopes I've used. They have all been exceptional. One really interesting function built in to the European models is the eye relief typically stays the same as the power is increased or decreased. Not so with the others.
However, even the 1" tubed "American" models tend to be heavier and much more bulky than Leupold, Burris, Bausch & Lomb, etc...
As for quanitifiable difference in the field for clarity, brightness or dependability, the 30mm tubed European scopes tend to be a little brighter, but cannot tell any difference otherwise. One thing about the brightness factor. In many states, at least for big game hunting, legal shooting hours are from 1/2 hour before sunrise to 1/2 after sunset. Some of the big European scopes will take you past that. My Swarovski Habicht 2,2-9x would.
The way I hunt dictates the scope should not detract from the operation or handling characteristics of the rifle it sits on. Therefore, all of my European scopes have been taken off the rifles they were on and replaced with Leupold and Bausch & Lomb. Well... Except for the Kahles on the 500 Jeffery.
I found the Zeiss and Swarovski (1" and 30mm tube models) aperature bells to be too large for the pre-64 M70 Winchesters, at least those chambered for magnum length cartridges. The bolt handle will actually rub against the side of the scope as the bolt is drawn back. This doesn't happen with the Leupold or B&L scopes.
To be honest, the European scopes are too big, bulky and heavy for the way I hunt. They alter the handling characteristics of the rifle they are mounted on (handling characteristics are the main factor in my gun buying decision making process) to the point I become less effective. Leupold's brightness, clarity and dependability is equal to or very nearly equal to those European scopes I've owned. Bausch and Lomb (or today's Bushnell Elites) are a good choice also and are as streamlined as the Leupolds.