Thunderbird is great as a cross-platform mail client; I run it on FreeBSD, Linux, and Windows. The Windows version has a slightly different menu layout, but I don't really mind this and it rarely comes up in regular use anyhow.
I do however have two gripes:
1) When saving an e-mail as a draft, and then re-opening it another machine, it often changes the From mail account. As such, I end up sending mail from my personal mail account when intending to use my business mail account. This was reported as a bug years ago, but has still yet to be fixed.
2) When logged into the same IMAP account from several machines, sometimes a mailbox created on one machine doesn't show up on another, and the only way to get it to show up is to toggle "show subscribed folders only", restart Thunderbird, toggle it back, and restart again.
There was already an existing bug report in Bugzilla when I first ran into issue 1) years ago, and I added my comments to it.
Issue 2) I haven't. As annoying as it is when it does happen, it's not that common and although requires some extra steps, doesn't result in looking unprofessional to clients like in 1). However, there has been a few instances lately where my staff have created new subfolders within a shared imap folder and moved e-mails into them, and I've been unable to find those emails until I recall this issue. So it's probably worthwhile to start gathering details and file a report to prevent this interruption in workflow.
I do however have two gripes:
1) When saving an e-mail as a draft, and then re-opening it another machine, it often changes the From mail account. As such, I end up sending mail from my personal mail account when intending to use my business mail account. This was reported as a bug years ago, but has still yet to be fixed.
2) When logged into the same IMAP account from several machines, sometimes a mailbox created on one machine doesn't show up on another, and the only way to get it to show up is to toggle "show subscribed folders only", restart Thunderbird, toggle it back, and restart again.