I am one of those backward souls still using a standalone e-mail client (Mozilla Thunderbird) for most of my e-mail reading and sending. My three main reasons are:
- good Afrikaans localisation in Thunderbird, which GMail still lacks;
- using a standard protocol for offline copies, allowing me easy offline access (which I know GMail now kind-of has); and
- the range of add-ons for Thunderbird, with some nifty productivity tools.
That said, GMail is the subject of constant improvement and I am a big fan of the nice new themes and the option "Include original attachments" which has recently appeared and often switch to the GMail interface just to use the latter.
GMail has also recently made it possible to select which labels (hence, folders) are visible to the IMAP server. Although one could also manage this using the IMAP client's Subscribe feature, I like the clean look that is now automatically there when I use any IMAP client to access my GMail inbox. This feature requires (free) subscription to the GMail "Lab features".