
The Locke/Eko dynamic is one that I've really come to appreciate on the Redux rewatch. To me they represent two distinct — but not mutually exclusive — spiritual paths that many people adopt in their lives. One tends to be literal and structured (Eko), the other tends to be more visceral (Locke), but there's plenty of overlap in between.
Lindelof and Cuse highlight these distinctions in "?". The episode kicks off with Eko having a bizarre dream, one in which he learns of Ana Lucia's fate and speaks with Yemi. Yemi urges him to help Locke, for Locke's faith has been severely crippled after several conversations with Henry Gale.
Tending to Libby keeps Jack, Kate, and Sawyer occupied in the Swan for much of the episode, allowing Eko and Locke to team up in the jungle. After a contentious trek to the beechcraft and another dream sequence, Locke and Eko discover the Pearl station.
What's ironic about this is that Eko was tasked with helping Locke, yet the Pearl did anything BUT restore Locke's faith. Meanwhile, Eko's is inexplicably rejuvenated despite the content of the Pearl orientation video.
The flashback storyline gives the audience some clues as to why Eko remains steadfast in his faith. After investigating what appears to be a non-miracle, the subject of that investigation (Charlotte Malkin) claims to have a message from someone she met while "between places". As luck would have it, that message comes from Yemi.
Major questions have been introduced: was the Swan really the site of a social experiment? Was it the Pearl? What could the DHARMA Initiative possibly gain by having people observe and take notes, only to have those notes vacuumed up to oblivion?
Perhaps the title of this episode is more appropriate than I even realized, for it's exactly what floats above my head when I try to understand the logistics of the Pearl. Later it's revealed by that the Pearl's actually monitoring multiple stations, but that doesn't necessarily clarify which station(s) Dr. Chang was referring to in the orientation film.
Minor deductions for Lindelof and Cuse taking great liberties with conveniences. I wish they'd found a way to advance Locke and Eko without resorting to dreams, but it's not a deal breaker. It's also a stretch of credibility to think Charlotte Malkin could've gone to the airport and met Eko considering her father was so overbearing and protective.

I personally think it's a lot of fun to watch Adewale Akinnuoye-Agbaje and Terry O'Quinn together, for whatever AAA might lack in emotional range TOQ certainly offsets. To be clear, that's not a slam on AAA and I think he plays Eko quite effectively.
In the beginning of the episode we see a fairly lighthearted moment with Eko chopping wood. Naturally that moment is soured just seconds later when he realizes Ana Lucia's dead, but it was fun to see Eko smile nevertheless. Perhaps a few more moments like that would've made him a bit more endearing and deepened the impact of "The Cost of Living".
Bonus points to Cynthia Watros for her excruciatingly realistic struggle to fend off the inevitable. The moment she coughs up blood always catches me off-guard and the sound is bone-chilling.
Average marks for Michelle Rodriguez, Jorge Garcia, and the guest cast.

This was the only directing effort on LOST for Deran Sarafian, and I'm always impressed how people can slip into the show and direct so effectively without compromising the spirit of LOST.
Both dream sequences were skillfully shot and edited, even if Michelle Rodriguez's stiff dialogue and hand gesturing almost sabotaged the first. I loved the sprawling shot over Eko's shoulder as he stood atop the cliff. The gradual panning to reveal the question mark was an exciting moment indeed, as was the slow reveal of the Pearl's inner chamber.
Sick as it sounds, Libby coughing up blood was impressively realistic. This isn't the A-Team where everyone escapes from gunfire and fiery car crashes unscathed — when people take bullets in LOST it's not a pretty sight.
Bonus points for the CGI on the bird's-eye view of the Pearl entrance.

There's no question (pun intended) that the discovery of the Pearl has a ripple effect on Seasons 2 and 3, leading to the discovery of the Flame among other things.
"?" is also important in that it further obliterates Locke's faith and sense of purpose, prompting him to reject the job at the Swan and ultimately leading to the station's destruction.
There's definitely replay value in watching the Locke/Eko dynamic if you're not opposed to spiritual subject matter. There's plenty of activity in the Swan and the jungle which helps with longevity, though I personally thought the Malkin/flashback storyline was a bit dull.

This won't go over well with Hurley fans but I'd be lying if I didn't say it: the emotional impact of Libby's death was almost single-handedly negated by Jorge Garcia. If an actor can't force out a tear then it's wise to convey the emotion some other way. Director Deran Sarafian might have been better off with a wide shot of a silhouetted Hurley kneeling next to her, then letting the audience fill in the rest.
In fairness to Jorge I did get choked up at his line to Michael: "I'm glad you're okay, man." This was a case where Hurley's innocence augmented the emotion of the scene.
It's no surprise that a Locke quote captures the most emotional moment of the episode: "Every single second of my pathetic little life is as useless as that button!"