Bought > Sold > ? > Bough again > Sold again
I know what you're talking about.
So far, over the last 8 years, here's the list of 50mm lenses I've owned/sold (in chronological order):
Canon f/1.4 USM
Zeiss Planar f/1.4 C/Y mount (Bob Shell adapter)
Canon f/1.8 II
Zeiss Planar f/1.4 ZE
Got my eyes open for a Canon nFD f/1.2 now that EdMika is making really nice adapters for them. I still have the ZE and nifty fifty (technically it belongs to the GF), but I get the feeling that the ZE will get sold if I get the nFD f/1.2.
I don't know whether it's my copy or what, but the ZE seems to lose a massive amount of contrast when used wide open (the C/Y Planar certainly didn't. I regret selling that one; it was before I had my Katz Eye screen that enabled accurate manual focus). After f/2 or so it's razor sharp & super nice, but it's got a weird look to it when used wide-open, which is primarily what I bought it for. Photozone & other tests indicate a noticable loss of contrast at wide apertures, but not as bad as what I'm seeing... I've also got a Voigtländer 20mm Color Skopar pancake that's pretty much made on the same production lines & it's great wide-open (granted that's f/3.5, but still...)