I like Phoenix Mail. It's just plain mail, and small too!
Still, I think it wouldn't be a massive succes since most people want HTML
email.
Besides, Phoenix can be hard to handle for less experienced people.
I haven't tried Pegasus.
By the way, I also use Mailmoa, which is good for 'fast handling'.
Mailmoa can be found at
www.moazon.com, I've sent them a Dutch translation
file long ago but haven't got a reply.
I guess the development has stopped, but it still is a nice one.