I’ve been spending some time with the Voigtlander 28mm f/2 APO-Lanthar to see if it actually lives up to the hype of being a 'Summicron killer.' While the price gap is huge, the performance gap is surprisingly small and in some areas like CA correction, the APO glass actually takes the lead. I put together a review on Pure Rangefinder covering the rendering, build quality, and how it handles on a Leica body. If you're weighing this against a 28mm Cron or even the Ultron, hopefully, this helps your decision!
https://www.purerangefinder.com/blog/8/better-than-a-summicron-voigtlander-28mm-f2-apo-lanthar-review