I think the slowdown in email clients to a crawl is a real problem. Gmail is great for some stuff (e.g. I have an account for my programming-related personal mail so I can access it anywhere, and I auto-forward each mail to my ISP account).
However for heavy volume usage, such as email forums, nothing is remotely close to an email client filtering into dozens or hundreds of boxes, having many views, having at-home composing, as some of the posts pointed out. Eudora has been mine for years, noting the stability and industrial-strength filtering and very helpful user forums (when I looked at Thunderbird a while back the filtering was like toy-level, may be better by now). I would always consider Becky, TheBat! and Poco as well as some others, especially now that Eudora has stopped development of the base product and been sort of absorded by Thunderbird.
However none of the alternatives has inspired great confidence (not to say they aren't good products, but I always would run into weaknesses) so I stick with Eudora while agreeing that the lack of the competition of a few years back has stifled development. Gmail on one end, Outlook this and that on the other.
I'll await and await. (Eudora 7 is fine meanwhile.)
There is a company that is trying to be a sucessor to Eudora, Infinity Data Systems, with a product called "Odysseus", billed as "The Eudora Successor". Perhaps it is in Beta, maybe. Too soon to tell if it will pan out.