I like having my premium membership. It lets me do fun polls and it's a lot easier to browse stuff on dA when you're not clicking 'next' page every three seconds. It gives you more flexibility when it comes to customizing your page, and if you like writing lots of journals the custom skins feature is quite exciting. Also, if you want to change your name then I understand premium memberships are nice to have!
I think it really just comes down to the amount of money you're spending. A three month (Or, in the case of this deal, a 6-month) membership is 636 points, which is, like, 8 CAD or something. Not a huge amount of money - most people spend more than that on coffee in a week.
In conclusion, I think it enhances the quality of your experience here on dA. If you're considering buying it, do it - take advantage of this deal while it's still happening.