(Preliminary note: I mainly work with Windows. On other platforms I might probably come to a different conclusion.)
My first half-decent mail client was Mozilla Thunderbird version 0.something. (Early adopters, anyone?) Before they came up with it, I had been using Outlook Express and similar clients. I just did not really use e-mail back in the days. I was rather contented with Mozilla Thunderbird, it did what it should, it was free, it was convenient and it did not even stumble about my preference for weird server configurations. (Don't ask me why. I seem to have no luck with my chosen hosters.) Moreover it allowed my to use GnuPG and NNTP which went very well with my commitment to the German Pirate Party and similar occasions.
Then the problems began.
Suddenly Thunderbird turned out to be fractious about IMAP management. The faster update cycle bothered Enigmail so it broke every few weeks. Also the application felt quite sedate at times. Previously appreciated features - e.g. the possibility to show/hide e-mail headers dynamically - disappeared from the core application and had to be added via third-party extensions. The fact that now and then there were essential improvements among the changes, like the new user interface of Thunderbird 17, did not compensate that for me.
One fine day after Thunderbird 11 or something I accepted that a replacement was needed. However, to find a decent one proved to be very difficult. The first result, due to convenience reasons, was to drop web mail services off my list of potential replacements. I have to manage more than ten separate IMAP accounts by now - try to manage them per web mail clients. (And don't even dare to throw in Google Mail, that ads-partner-polluted piece of something. Aside from my sane paranoia about Google's evilness: I would really miss the convenience of a decent desktop mail client. Again: A certain number of IMAP accounts with very different configurations are soliciting my more or less regular attention.)
My list of requirements for a decent replacement was rather short: GnuPG 2 support and a threaded view (for my subscribed mailing lists) were quite the only needed features. NNTP was optional, I could as well use Opera, still Thunderbird, SeaMonkey or the like for that. (I don't know whether SeaMonkey can handle GnuPG 2 or not - on the other hand I never really was into the Mozilla Suite either. I considered it - and Opera - too hard to use because of the different moduses - mail, browser, ... - when I only need one.)
The choice (this is a good moment to remind you that I primarily use Windows) was appropriately complicated:
- I generally like Pegasus Mail but it crashes reproducably - I had reported the issue, but AFAICS it has not been fixed for months. Also the handling needs getting used to for a while.
- Outlook has an awful user interface. No-go: No support for OpenPGP/GnuPG available (or would it require obscure plug-ins or something?), so it's out.
- Claws Mail seems to be something like Thunderbird in hideous clothes. Also it can't work with HTML mails. ;-) (Don't take this too seriously.)
The consequence was my union with a good old friend, enter The Bat!. It can do anything I need and had been developed continuously for years now. Using the trial version was - apart from initial weirdness about using CA certificates which are monitored internally by The Bat! - almost fun to me, GnuPG 2 works out of the box and the templating system (you can define complete templates for new e-mails, replies et al.) are for power mailers like me a must-have. You know you need it when you use it for the first time.
The Bat! was well worth the (reduced) ~ 20 € for a full-featured Professional license (valid until version 6.0.99). I also get a Voyager (portable The Bat!) with the license, the very helpful and kind German community is one more reason to like it. The developers (RITLabs, a Moldovan company) replies to bug reports quite fast and fixes severe bugs in one of the following beta versions if possible. Also included: Profile encryption, schedulable backups of the complete application with all accounts, import from Thunderbird.
Of course The Bat! is mainly a mailing application. No NNTP, no RSS, only a rudimentary calendar without cloud synchronization. - Anyway, if you are a power user of RSS and/or calendars, you probably already use (like me) dedicated solutions. Compared to FeedDemon/RSSOwl and Rainlendar, Thunderbird's provided functions are sort of a joke.
As a side note here's some screenshot after having moved all mail accounts from Thunderbird into The Bat!:
Je ne regrette rien.
You are kindly allowed to make fun of me now.