Legion is definitely worth getting back into the game for! My guild fell apart with Cataclysm and most of my friends left and I was just bitter over WoW for a long time and only played each expansion long enough to get a character to max level. For the most part, I felt like the expansions weren't very fun - Pandaria was interesting storywise but boring for everything else, WoD got a bit overdone (and while I liked garrisons, they felt awful after so long.)

But Legion has been different - while there is grinding, it doesn't feel like the endless grinding of WoD, demon hunters are awesome, the story is pretty solid, and the zones all have enough going on to make leveling easy. You'll get a character boost with the expansion so you can dive right into Legion content too. If you don't want to buy right away, I'd recommend waiting until Black Friday sales - Legion is sure to go on sale, so the initial investment won't be as much. Also seconding the anon above me - class halls will spit gold at you, and world quests often have gold rewards.