From what I've seen, a lot of people don't like Across the Sea.
I actually liked it (not in its entirety, but most of it).
I don't necessarily blame those who don't like it; to me it seemed done in a different style than previous episodes, and not necessarily in a good way. People say that the explanations offered in the episode were weak or cop outs. While I don't entirely agree, I can definitely see how it would be off-putting to some fans. It seems the thing most people dislike is the backstory as to why MIB is so eager to leave the island. People were expecting some much more sinister motive instead of a desire to run away. People also didn't like to see Jacob as a mama's boy (let's be honest here, he was), and finally people really didn't like the supposed origin of the smoke monster.
I'm probably the only person who thinks this, but I liked seeing how Jacob isn't omnipotent and how MIB's motives weren't all that sinister. It's good to see how theyr'e human after all, kind of like Ben's story last season. To me it reinforces one of the underlying themes throughout the series: people aren't perfect. Time and time again, through the flashbacks and flashforwards, we've seen how the main characters led broken lives. I think it's good to see "autority figures" such as Jacob and Ben broken down into the people they once were; it reminds us they're not gods and were not always manipulative and devious, and that maybe they didn't want to end up like they did.
The humanization of Jacob and MIB also reinforced the idea of subjective good and evil that I've been subscribing to for, well, ever... I posted another blog on the subject earlier in the season when Jacob's and MIB's motives seemed more dichotomously good and evil. Now, seeing how they once were, we can see how Jacob and MIB aren't all good or bad. Most importantly, we see Jacob starting fights with MIB, which is hardly a thing a "good" person would do.
It also seems to me like the smoke monster may have existed before MIB. Here's why: MIB's corpse was laid along his mother's and was discovered so many years later by Jack. And yet, we see him later (in Ab Aeterno). Obviously, that was a physical manifestation of the smoke monster, but to me this seems as if the smoke monster took MIB's dead body in the same way as it took Christian's and Locke's. It also changed MIB's personality and made him more violent and manipulative, like it did Locke's. It may have just needed a body so it could take on physical form, instead of existing as pure smoke or pure energy in the Source. If it didn't exist beforehand, how else could Mother have known about what happens if someone entered the Source?
That being said, I didn't like the Source as a plot device. It seems too, well, fantastical. A magic spring that contains the essence of life? That seems tacky... I think that the writers probably could have come up with something a little better, but chose not to. Of course, there's probably a huge twist waiting to happen in the finale that will reveal the true nature of the Source, but until then, I don't like it. Like I said, it's tacky.