The main gain with a doing a partial or full mash is control, and getting a fresh malt/grain taste in the beer.
With extract, you get what you are given. You can alter some parameters, such as color and bitterness by blending different extracts and adding hops, but you get far more control when doing a mash. Also, you can mash ingredients that aren't available as extract, such as raw wheat.
With the mash, you can also control fermentability, setting the dial where you like on the dry-sweet/thin-full bodied scale. Finally, doing a mash just smells wonderful! It makes you really feel like you are making beer.
But whether these things make doing a mash worthwhile compared to extract, well, only you can decide that. I think it is, as to many all-grain brewers, but everyone will have their own opinion as to what is the best time/flavor tradeoff.