Well yes, it's actually been going very well. Thunderbird seems much better than 4 years ago when I last used it full time. I have not used kmail now for around 3 weeks.
I am using TBird to connect to five IMAP accounts. Of these one is quite high volume with 100 emails in and out each working day, many with attachments. But TBird seems to be handling them all OK. I have set TBird to sync all emails locally as I find it really speed things up. This did take a long time as one of the email accounts is 30Gb +.
I find I now have confidence with my email. Before with kmail, I often used to send myself an email just to see if it was working. And soemtimes it wasn't. But TBird seems almost 100% reliable so far.
My one concern is the size of my local mbox files. Some of these are 6Gb + and it would be annoying to have to re-download these if they got corrutped or damaged. I do back them up everyday so that should help if there is a problem.
I have also tried TBird in maildir mode. (There is a setting in about:config). However, I am not convinced this works properly yet as I kept getting empty folders and zero byte files in the maildir directory. So I have not persued this.
Next I looked at offlineimap. This syncs an imap account to a local maildir. I tried this on one of my smaller email accounts, but of course I then realised TBird will not read maildir.
The other thing offlineimap can do is sync a remote imap account to a local one, so I installed dovecot on my local machine, did a bit of googling around to get it working, and now I have offlineimap syncing my remote imap locally to dovecot and then I point TBird at the local dovecot server. So far this is working quite well, and all my local emails are in maildir format.
Anyway, at the moment, very happy with Thunderbird with this last set up. I certainly have no plans to revert to kmail. It's a shame to say this because kmail is almost a very good email client.