I bought Obsession on release. It's good, though my enthusiasm has waned with some of the usability/oddities it has. The core sound is great. I've been using bx_oberhausen more again as well, and might like it just much as Obsession in different ways - that's a steal for < $30.
I tried Knifoneum and have already moved on. It just wasn't anything interesting or different enough that I'd want to try and use, or perhaps took too much work to get there from scratch. So... I finally purchased TAL-U-NO-LX v2 instead after trying K. I don't expect to buy K, even once it's down to ~$25 (it'd be $75 for me to buy it right now).
I applaud PA for creating boutique style software synths, though the CPU usage isn't awesome. From what I can tell, it's challenging to create a soft synth that sounds really really good. Especially one modeling analog. PA rose to this challenge. It's then just as hard again on top of that to make something that sounds amazing AND doesn't chew up a CPU as much - it even took U-He years to figure out how to best optimize Diva, for example. I don't expect PA to continue to optimize for long, unfortunately, as they have more plugins to make and market.