I use one with my X2 - it is a nice finder and it works well. You just need to remember where the AF patch would be in your composition. Also, it is worth temporarily taping over the connnector socket to avoid getting dirt on the contacts.
Just browsing and looking for an external 35mm finder for a Barnack. Will the Sony FDA-V1K work on a film Leica? [I know, it's blasphemy, but I would like a Zeiss finder with the wonderful glass.] Any help appreciated.