I don't get all the high ratings and raving reviews this episode has gotten here (but then I'm often perplexed in that regard). In my opinion, this is a weak, derivative, poorly scripted and badly characterised episode that commits the cardinal sin of being *boring*. It combines two Trekkian cliches that have been utterly done to death: the space anomaly and the omnipotent alien that's curious about humans (god only knows why we're so fascinating to advanced life forms).
The teaser to this episode is utterly pointless and completely unrelated to the rest of the episode. I can only imagine it was tacked on because the episode was running short (because, let's face it, there isn't exactly much plot). Picard is sitting on the bridge worrying like a mother hen while Riker joins Worf in his exercise program. The worst characterisation in this episode is Worf, who is portrayed as being a volatile rotweiller who can't control his aggression and nearly turns on Riker twice. This guy got through Starfleet HOW??
The rest of the episode is basically a snooze fest. There are certainly some interesting moments, including some genuinely eerie scenes aboard the Yamato and Picard's musings on death, but I really didn't understand or buy into his decision to destroy the ship. The ending, where Picard holds off aborting the self-destruct until the absolute last minute is a silly and desperate attempt to inject some tension into the episode's (anti)climax. I mean, even if they hadn't really escaped Nagilum, why couldnt he just have initiated the self destruct again instead of dicing with death??
The conclusion, where we get Nagilum's assessment of humanity is not only a major case of deja vu (powerful alien condemning humanity as dangerous and savage, while Picard spouts on about how far we've come...yawn) but is also astoundingly hypocritical of Nagilum I would say.
Watch this episode only for the occasional unintentional laugh, such as Worf's scream aboard the Yamato, his occasional outbursts of Rotweiller growling and the amusing moment where Pulaski's body is taken over by Nagilum. Otherwise, avoid.