I use it. I have my gmail set up with it. I already had my school email forwarding to my gmail so I just left it like that instead of setting it up seperately. I just started using it when I got my mac to check it out and saw no reason to stop. I like being able to access emails while not online as well, for reference etc.. and the integration with OS X (in the dock, linked with address book and iCal, iPhoto etc for attachments).
I don't have a reason to not use it, though I'm not a power user who gets 100 emails a day from various sources so I'm not sure.