I don't have a Vortex but have checked them out quite a bit. I currently have a Zeiss Conquest 3-9X40 and love it. I did a side by comparison of the Vortex Diamondback HP 4-16X42 and the Zeiss 4.5-14X44. I set them both to 8 power and looked out the window at the store. It was pretty close, but I think the Vortex had a little bit better view. I would like to do a "blind" test for them just to see.
I have read a lot of customer reviews on the Diamondback HPs and have found nothing but good things on them. One thing to note though is that I found negative comments on the regular Diamondback's about short eye relief. This should be corrected on the HP versions as the regular Diamondbacks have 3.1-3.4 where the HP versions have 4.
I just got a new rifle.....25-06 and currently have a 300WSM that has the Zeiss on it. From what I've seen and read, I think I'm going to take the Zeiss and put it on the 25-06 and get a Vortex Diamondback HP 4-16X42 to put on the WSM.
One other thing about the Vortex is the unconditional lifetime warranty which may come in handy. I only have read one comment about that on another forum. A poster on that site (can't remember which one) said that his Vortex fogged up. He called the customer service dept. and they sent him out a new one with a prepaid carton to send the old one back. He didn't even have to send the old one in before they sent him out a new one. That's incredible.