20 is probably my second most used. The most is my MD 45/2. I also have noticed almost no focusing issues, but I did: put it on a Panasonic body, buy it new, and not overload it with hoods and things. It has had a UV filter since almost day one, and rarely a polarizor.
The 20mm focal length is an awkward focal length for me, so it's my least used lens. I've got the same problem with the Zeiss 35mm on my a7R. It's not long enough to be a "normal", and it's not short enough to be a wide angle. My most used lens is a toss up between the Panasonic 14-45mm and the Zeiss 80-200mm Vario Sonnar.
I like the 20mm its close to the 42mm on a lot of fixed lens rangefinders I have had fun using, it seems to focus a lot better on my G3's than the EPM2 where other lenses don't seem to notice. It is a superb quality lens so its worth putting up with its foibles for what it gives you.
I chose to buy the 17mm/1.8 at the time for the AF. I love it, it is my most used lens out of the 4 lenses I have. I can imagine the FL of the pany 20 would also be great. I might still get it one day just for the better IQ.
I find that I switch quite evenly between my 3 lenses (14, 20 and 45).
I tend to shoot with one for a few days, weeks before changing to another lens.
Brought all 3 lenses and EM-5 for a wedding over the weekend. After that, realised that I only switched the lens once at the end of the night to take a group shot with the 20mm. Was shooting all day with the 45mm.
One thing I really wished was that the 20mm's AF was quicker. There's a cashback promo available for the Oly 17mm in singapore at the moment, which makes it tempting for me to replace the 20mm with the 17mm/1.8.
However, there is also the Pana 15mm just around the corner. #FirstWorldProblems
My most used lens is my Olympus m.Zuiko 7-300mm F1.2 pancake zoom. I use it all the time when I'm dreaming...but then I wake up and find I have to make do with reality.
It that case I'd probably say my Panny 25 1.4.