I have one fairly major complaint about Gmail's web interface - you can't open multiple emails in tabs by middle-clicking. (Pot+kettle sidenote: one can't middle-click links in this weblog either, but that's an XSLT fault in Gecko.) In fact, it seems the “links” one clicks to open emails aren't really links at all (right-clicking offers no “Open Link in New Tab/Window” options). This would put me right off using Gmail permanently.
Other, minor complaints are: the clock is in 12-hour format; I prefer 24-hour format. Also, the ad-frame isn't Adblockable (but I don't expect Google to want to fix that :) ). The clock problem could be fixed easily, and neither of these are at all important (just nits).
But I no longer have to use the web interface. Although it is much, much, much, much, much quicker, easier and better than Hotmail's, Yahoo's and mail.com's put together, I just don't like web interfaces. Not when I've got an excellent email client.
For Hotmail I've been using MrPostman as a web-interface-to-POP3 proxy. For the most part, it works well, although not flawlessly - if and when your account gets clogged up with emails, MrPostman is seemingly incapable of ignoring the “Buy More Space!” adverts added to the page, insists you have no new mail, and refuses to fetch the non-existant mail. But it's workable.
MrPostman is aimed at Hotmail, Yahoo.com (not .co.uk) and mail.com - not Gmail. Bobbins.
However, it's a good thing that I read an inordinate amount of Mozilla-related weblogs, and have the middle-click-every-link-in-the-text affliction (“tabbed-browsing syndrome”). For cheeaun led me to djst, whose comments led to PGtGM - a web-to-POP3 proxy for Gmail! Incidentally, the blurb mentions Thunderbird by name - extra marks. (It should also be mentioned that PGtGM is only for Windows.)
How to set up Thunderbird (or even Lookout Express) to use PGtGM isn't made obvious, but it's there - in the PGtGM interface, click Help/About PGtGM, then click the E-mail Client Setup tab to the left of the dialogue. If you're using MrPostman alongside (as I am), you may have to change the POP3 port setting in both PGtGM and Tb's Account Settings for the appropriate account, to something other than MrPostman's. Et voila - Gmail via Thunderbird!