Like many, many other Lost fans I was concerned about how the writers would explain Michael's return, but it went off without any major issues. The flashbacks were effective, explanatory, interesting, and provided a chance to encounter some faces we haven't seen in a while. Harold Perrineau did a brilliant job this episode, proving that he is a great actor when given the right material to work with, which Elizabeth Sarnoff and Brian K. Vaughan provided with an excellent script. We see barely anything of 'present-time' island events, only getting the obligatory (but nicely done) scene with the rest of the 'important' members of Locke's camp learning that Michael was on the freighter, as well as Ben sending Rousseau to the temple with Alex and Karl, which led to the pretty shocking ending, which actually felt really out-of-the-blue to me, and almost disappointing in how it was written, though not necessarily for its content.
The episode's flashback story was nicely set up, with further mythology reveals (more like hints, admittedly) and nice character-driven moments. There's some great scenes here, and overall I'd say that it's around as emotional and interesting as "Special" from season one, which I personally thought was a great episode. Stephen Williams did a fairly good job of directing the episode and it was nice to revisit some of the music Giacchino wrote for Michael's character. All in all this isn't my favorite episode of season 4, but it is still excellent, and in keeping with the generally high quality of the season, excepting "Eggtown" and "The Other Woman", and to a lesser extent "Cabin Fever", I suppose.