I used to use Outlook with AIM plugged into IMAP. I attempted several times to get Gmail to run through POP and it didn't work. I can't stand google or gmail.
Gmail is good when it comes to forwarding, auto responses, and stuff like that where others are not there or I have to pay for it. But I had personal issues with their servers tracking my emails putting ads and them keeping my emails forever. There was too much contervesory with privacy and initially lacking a delete button that made me not use it for non essential emails. Then the beta thing isn't promising. It's now FOUR YEARS and they STILL haven't gotten out of beta. I used to remember that beta software only lasted 6 months to a year and a half before release. Google abuses the beta phrase. Gmail lacked IMAP support for too many years (was released late last year - too late), which was kinda retrotical because POP had taken me forever to download mail off my client because it downloaded the complete emails and not just the header like IMAP. IMAP is more broadband friendly and POP was designed for smaller boxes and dialup days.
Gmail is for punky, dumb, and hip people and doesn't give me a professional mail client (Outlook/Notes) like feel, and thats why Gmail hasn't changed their UI. Hell, I can't even drag a message into a folder like Yahoo does. And thats a perfect segway - I use Yahoo! Mail and I like it. I had liked it since the beta, and theirs was a faster beta to final version rollout. I wished the calendar and notes would be part of it though. Since the final Yahoo! Mail rollout, I never went back. Instead of using awkward opening and dropdown menus to move it, I can drag it.
For desktop client, again, I used to use Outlook on my XP partition on my Mac. My mac's wifi is messed up so I am on a desktop. I used to sync up my BlackBerry and use my AIM mail too. I am starting to grow out of Outlook and hopefully if I can have the ability to use my AIM mail in Lotus Notes as a stand alone client, I can't wait. I have it, but I screwed it up and I need Designer to fix the problem.
The best mail client, may not be Gmail, Yahoo, Notes or Outlook, it might be a BlackBerry. The BlackBerry can give you all your mail clients abilities on your palm but the only downside is emails are sent/received in ASCII (Plain Text) not HTML (HTML/Rich Text).