Indeed, I use my GMail also for forums, signups, etc..
For more personal stuff, like my job, and other important stuff, I use my dotMac. But I have to say that all my mail comes into my dotMac mailbox, because I've set up GMail to forward everything to dotMac.
I'm very happy with the whole dotMac stuff, It just works.
Prefs, mail, notes, bookmarks, keychains and more justs syncs between my iMac and Macbook Pro and its just a complete upgrade to my iPod Touch, without dotMac, the Touch experience would be much less..
But if you got only one mac, I'd say: just stick with gmail, you don't need more.