In my workplace (University Engineering School, ~300 users) we recommended Thunderbird for Windows users (although we allow them to install the one they choose) and I think we have had more than 40% share.

Two years ago our webmail system migrated to RoundCube (yay!) and since then, many people just used webmail. Many others, unfortunately, configured their (University) account in the Gmail webmail (IMO it should be forbidden to do that, specially for teachers/researchers, but it's difficult to get those policies written, and even if yes, more difficult get people following the policy).

This year I would say we keep at least 25% share in Windows desktops, but I have no actual figures (just estimating from the user help desk requests).

I've never seen a Windows user that migrates from other client to Thunderbird regret that decision later (except for the migration to webmail cases).