I've been using Claws Mail for the past few months. It's simple, fast and stable, and GPG support is fine. That's all I need. It sucks for reading and writing HTML emails, but I'm okay with that. Parts of the UI are still rough (e.g. the main window isn't responsive while the "sending" dialog is open), but it's less worse than Evolution.
We need entry-level e-mail clients to have GPG/PGP built in. Your post highlights the need to select a client which is why GPG/PGP has a niche user-base who send dull e-mail to Internet Celebrities who happen to share public keys. I mean my parents e-mail client, which would naturally be the default for their operating system (yes, Windows Live Mail), should have in-built GPG/PGP capability and the address book should auto encrypt any messages to me with the public key in my "contact" info, and decrypt any from me with their private key which is created and stored without them even realizing (as SSH does). It's hard to select an e-mail client anyway, but convincing my parents to select an e-mail client and/or plug-in, isn't going to happen.
I've been puzzled why fat clients traditionally have S/MIME built in, but certificates are complex to obtain, and don't have GPG/PGP with easy to create keys.
Strong crypto for emails has to work like https does for web browsing. Picture if people had to use some command line tool to generate and keep keys in order to do online shopping in their web browser. Compared to just using a (any) browser with https built in.
The amount of people that would want to use some native desktop email client for ALL their email work is likely even smaller than the number that use GPG today! If I couldn't read my email on my phone or on any computer it would have lost most of its value already.
While I really enjoy reading and sending encrypted emails, and think it's far better than sending sensitive data in the clear, I am surprised to learn that there are people who use PGP and don't hate it or have issues with it.
Enigmail is good though.