The original Limited lenses (FA 43mm f/1.9, FA 31mm f/1.8, and FA 77mm f/1.8) were all quite good, although I was never so fond of the 31mm as the other two. The FA77 was quite a wonderful lens, but the 43mm is just special. When I bought a K-01 last year, I immediately bought another 43 Limited to use with it. And it's the only Pentax lens that I could see buying an adapter for to use with the SL. The FA43mm Limited is just that good.
Pentax sold it as a truly "limited edition" Limited in Leica thread mount with RF coupling and dedicated optical viewfinder because of the demand for it. It's that good.
I wouldn't exactly call it a "pancake" lens. It's more akin to the Mandler design Summilux 35mm f/1.4 v2 for Leica M in size and overall shape. It's a near-symmetrical design and relatively short because the SLR body is deep.
The FA43 is my personal favorite Pentax lens... I essentially bought the K-01 just to have a native body to use it on, and re-bought the lens since I'd sold my original copy when I opted out of Pentax equipment some time back. :-)
Oh yeah ... K-1 ... Looks nice, typically Pentax it jams a lot of stuff in for the money. I don't know what the big deal with that wacky LCD articulation mechanism is all about, looks too complicated and fussy to me. But otherwise, if you've got good Pentax lenses, it should be a good camera.