My own thoughts (Immortal, mostly normal AI so far, and I haven't played with Solver's patch yet): The AI does tech more slowly, often significantly so. It also expands to fill the land more slowly. If I can block off enough land for 8+ cities, I can actually get ahead fairly easily, and either fight an advantageous later war or simply play for a totally peaceful spacerace. However, taking land by warfare has become very much harder. The AI now knows how to surge its production via whipping, and use its defenders/counterattackers more effectively. Plus the nerfs to siege units.
Overall, BtS seems to be easier than Warlords if I get a starting location with enough room to expand. But the variance is also higher -- a boxed-in start, with room for just 3 cities, can be very difficult (whereas in Warlords, it generally wasn't too hard to fight my way out, with catapults if no metal was available). I'm not sure I like the overall shift. I like variance in gameplay (which strategy will work best varies from game to game), but not necessarily in difficulty. I'm afraid there will be too many "yawn, another trivial victory" and "damn, that was hopeless" games, neither of which I enjoy.
For now, I'm trying modestly overcrowded maps (12 total civs on large, instead of the default 9) and experimenting with Agressive AI, trying to find a balance that I like.
peace,
lilnev