Both the expansions are 150% worth getting. Ashes adds a huge new campaign and the Inquisitor & Necromancer masteries - Inquisitor is a great secondary to many other masteries and Necromancer is an amazing primary. Forgotten Gods is a shorter campaign and more of a side story, but the Oathkeeper mastery it adds works very well as both a primary and secondary, and it adds the shattered realm, which functions as an alternate endgame (as well as an efficient stepping stone to get there, and a testing ground for new builds). Both expansions add a tonne more items, with AoM raising the level cap from 85 to 100 (you must have AoM for Forgotten Gods to function)

Personally, I've logged something like 2.5 thousand hours in the game, and i haven't exhausted its potential yet. Just so you get an idea of what you're getting.