I think Seasons is must buy. It doesn't seem like it right away, especially if you start in summer or spring, but it slowly shows itself.
Where expansions like Cats and Dogs or City Living showed you everything new right off the bat, and you could jump into them and explore everything, Seasons adds overall quality to your whole game.
I feel like expansions like City Living are almost stand alone. If you don't live in the city/go to the festivals it adds barely anything to your game. Seasons, on the other hand, adds quality to your game everywhere. It ramps things up in every facet of your game.
Holidays are great! I love that you can customize holidays and make your own. Be warned though, they can take your vacation days without you knowing. Any holiday where it says "day off school/work" WILL take a vacation day.
Weather always makes everything more dynamic too. Even just seeing rain outside makes it feel more alive.
Gardening has been changed and I assume for the better, but as I'm currently playing a legacy and as per rules I won't have a gardener sim for a few more generations, I haven't got to try out the new system.
But there is a sprinkler! Which is just amazing. No more watering all those plants.
Overall, I consider Seasons a must buy.