Wow is absolutely worth buying.
At no point was the game bad or unfun. The game has had a ton of flaws introduced to it over the years, that's for sure, but most of those flaws are only highlighted due to the way the game was years ago. However, the game remains insanely good in spite of these flaws, and it's also just a fact that many people -like- what blizzard has done over the years, even if I don't. (things like reducing the exp needed to level drastically, and damaging immersion for the sake of convenience, contentious points that are good to some people and bad to others.)
Aside from these contentious points, the game has objectively gotten better over the years in other ways that even the most nostalgic players can't deny. (Like raids are insanely better these days, and the questlines on the latest expansions are so good...)
So yeah, it's worth buying. The real question will be if you think it's worth the sub fee, but personally, I think it is as well. I take breaks from time to time, but every time I come back, it's always for like 6+ months and I enjoy it the whole time.