Overall, fairly happy. With every positive, there are some negatives, but I do think it's been good overall.
I like having new monthly content to explore over time - I do feel it has you coming back to try things out again as you need to explore the new changes. Downside? I always get that annoying time like a week before the patch where I don't want to start a new game since I'm waiting for the updates, but I finished my previous game. Also, at times, it can get annoying how the "game rules" change every month, so you have to learn something new.
New civs have been fun overall. I have mostly enjoyed the new civs, and while most are certainly on the stronger half, they've at least given a more or less unique style to them.
On the game modes, they're a good way to refresh the game and let you try a completely different game. Being able to choose which modes you want on or off gives you even more variety beyond the basic civ/map choices. The downside is that they are very strong, very unbalanced, and the AI troubles with some of them generally make the game much easier with them on than otherwise. Also, I don't like how a majority of the modes are still more on the "fantasy" side of things, rather than pieces like the Dramatic Ages which take a current mode and basically alter how it plays out.
Do I like it more than had we simply been given a full expansion pack with all the content at once? I don't know. Maybe? Hard to say, and the big problem with a game of this size, it all gets mixed up together. Like, when I'm playing, I totally forget that something like the Diplomatic Quarter was simply something that did not exist before July. Still seems weird to think about stuff like that...