I've shot both the 28mm Zeiss Biogon ZM and the 28mm Viogtlander Ultron VM, just not at the same time.
When the Voigtlander first came out I was sent a copy to test against my Leica 28mm Summicron-M. Of course, the Leica blew the Ultron away - but not wide open (where they were very close). However, as they were stopped down the Leica improved dramatically, while the Voigtlander did not.
I've owned the 28mm Zeiss Biogon ZM and was never dissatisfied with its performance. I only sold it because I also have the 25mm Zeiss Biogon ZM, which marginally outperforms the 28mm ZM stop-to-stop and I like the extra room in the corners the 25mm FL provides.
In summary, I can say the Voitlander is very consistent stop-to-stop, but mediocre at its optimum apertures compared to its Zeiss competitor. If it had some "character" to compensate for not being particularly harp, it'd be different, but as it is, I personally cannot recommend the Voigtlander - even for the extra stop it provides.
To be reasonable though, is it "sharp enough" for most shooters? Probably - as long as you don't try comparing it to the Zeiss.
