Interesting comments about the different Zeiss lens so far. While I've hired the 21mm on occasion, the 15mm was the one I bought, mainly for big, night sky shots. The 21mm just wasn't wide enough. That said, the 21mm had some great uses for landscapes, in a limited fashion. Both have proven to me to be sharp at f2.8, except at the extreme corners.
If I get the chance, which I might next year, I'd like to get the 85mm and maybe the 21mm.