It's a funny thing - and I don't even understand why myself - that the Desmond-centric episodes are always the ones with the greatest potential to mix head-spinning mythology and old-fashioned human romance and drama in perfect measure. This episode joins season 4's "The Constant" as two of my favourites, mixing both WOW! moments with tears of emotion.
I'm guessing most other reviews will go on and on about the mythology-element, and how this episode marks the beginning of the two realities merging, so I'll focus instead on the character side of things. There were so many returns of sorely-missed faces from the past, from Charlie (whose chemistry with Desmond remains intact) to Eloise and even Minkowski - and, of course, Penny. Sonya Walger and Henry Ian Cusick go down as one of television's greatest couples, with that final scene (despite the only one in which Penny appears) the culmination of an episode's discussion of whether fate and destiny can keep true love apart. The answer? No. And thank goodness.
Otherwise, we get the WOW moments of when Desmond disappears from the main reality between those two gigantic spinning pillars of energy, and that stinging flash to Charlie's drowning, to those climactic moments at the end where Sayid "rescues" Desmond, and alt-Desmond demands the Flight 815 passenger list so he can spread the truth about the other reality they should know about.
Next week looks like the last single-character-centric flash-sideways, and Hurley is usually good value, but to be honest I'm more looking forward to the following week when we'll probably start seeing all the characters and situations come together in both realities - and we might finally get the answers to the fundamental questions of "Lost".