I've never liked Kmail. For simple pop/imap mail, it's simply too large, complex, and requires too many things enabled on the back end that I don't like to bother with. The fact that it's also uglier IMO than Thunderbird or Evolution is just further reason to remove it from any system I use.

I have tried to use Kmail in the past, but I never found it really convenient. I have been using Evolution
on Debian until 2010 and I switched to Thunderbird on Slackware after that. In both cases, I was using them
with an IMAP account. If you need a lighter email program with IMAP support, you could use pine which is
also easy to configure. If you have enough memory, it is better to stick to Thunderbird.
If you need to have a calendar function to schedule meetings, you may use the Lightning extension with Thunderbird.

I agree with most of the posters here. I've tried it in the past and found it to be hard to set up, and to offer no real advantages over Thunderbird. Plus I tend to bounce between desktop environments every few months and Kmail isn't really happy outside of KDE.

I've had no issues with it. I access a Dovecot IMAP server that I personally control.

Perhaps my e-mail needs are minimal.

Claws-mail and IMAP does the trick for me. Claws-mail has a rich feature set and a low memory footprint.

You can have alarm with Thunderbird. Install the Lightning plugin, define an event and activate the "remind me..." function.

I have multiple e-mail accounts that I use, as well as an mbox archive (thousands of mails, dating back to 2005). I hadn't tried kmail for 5 years (because I didn't have a positive experience) until a couple of weeks ago. Kmail imported that archive just fine, but everytime I clicked on that folder it'd take a few minutes just to display the contents of the folder. It seems like Kmail for some reason wants to parse all these e-mails every time you click on the folder. With Thunderbird this is instant. If you add the other quirks here and there, and a couple of crashes (I've never had a single crash with Thunderbird)... I'll probably give Kmail another chance in 5 years.

The only time I successfully used KMail was with a Dovecot-offlineimap setup that dumped mail into a maildir. KMail used that maildir quite happily.

That combo was useful with GMail since it eliminated the delays I saw with their IMAP implementation. When I moved to Fastmail, I noticed those delays had vanished and I dumped Dovecot and offlineimap and started using Thunderbird.

I used to love KMail before they transitioned to akonadi & friends (KDE 3.x, and earlier KDE4 versions). For me it was by far the most convenient mail client (despise it not being able to compose or forward HTML mail).

When I had to move my main machine to Windows, I really missed it. Installed Thunderbird, but never came close to the same feeling of pleasant ease of use (I don't really like the whole "we don't do x, install an extension" thing for basic stuff like minimizing to the system tray, or the constant updates for things I don't really need), at the end I ditched thunderbird and moved to windows live mail and outlook.com, with my archive mail stored in a local IMAP server).
I tried to use the new KMail2 it in a VM some time ago, and it is just not the same thing anymore. If it had been the same I remember, I would probably have used it even if it meant to have a VM open only for it. But it has become a pain to setup, and it is much more unstable than what it used to be.

KMail2 has not improved over what we had in KMail before, and sadly the same thing is happening a lot in the free software world.