Very reliable. Strip clip loading into (fixed and detachable) magazines was common in WW2. The Vz. 58 is unusual for carrying on the feature into the assault rifle era.
Yes, this is still the ideal way to refill magazines. Though the unique part of the Vz.58 is being able to refill with the clips directly into the rifle, rather than detaching the magazine to refill them. The rifle has a built in clip guide.
Perhaps I’ll show off a handheld 5.56mm clip guide at some point.