Tough call as far as I'm concerned... When I watched Berserk I didn't know much about it, so I went in without paying too much attention at first and didn't have any preconceived thoughts, by the time I was in the middle of the series I had almost totally forgotten about the first episode, so the ending was like a huge plot twist and shock for me. It worked the same way with everyone I showed the series to. I know it sounds weird and doesn't really make sense, but that's exactly what happened, and it's more puzzling that it happened to my friends too... But when we go back and watch the first episode again after finishing the series, we're dumbstruck thinking "How did we NOT see it coming?"
But your case will likely be different, since you already know about it and you're asking this, you'll probably be fixated on the first episode and try to make predictions along the way, thus lessening the impact of the ending.
In either case you're in for one hell of an anime, Berserk for me isn't about the action, violence and battles, or even the ending... Its strongest point is character development.