I use Gmail for some special accounts, and forward the incoming email to also go to my Inbox download. For large quantities of mail, such as internet forums, I find gmail far too cumbersome and I do not even consider using it. I use it more for some personal home-biz mail where I may be at work away from my downloads and I want to be able to check easily at home or work.
If you want to stick lots of mail on your disk for with quick filtering and searching and moving and deleting, I find Eudora (or another comparable client) the only way to go. Despite all the brouhaha about gmail and other webmail.
Also for composing email there is no comparison. The stability and speed and more of composing at home is far, far better than the net. Again, I only compose on the net if I am at work or traveling. (Even then I could compose with Eudora or Thunderbird, however I do not want to bother with SMTP so you run into some awkwardnesses in long term archiving, so I compose with my Gmail account, which I find .. acceptable.)