In its time, Phantoms was reportedly considered unimaginative. I consider that pretty harsh; it may be another movie about some people in a deserted location with a monster, but those behind it at least made their own interesting take regarding this scenario.
Too many horror movies these days - especcially big-budget horror films, for which its a ridiculously high number of non-sequels/remakes - are highly derivative of either The Ring, Saw or build around one of the main ingredients of Saw (excessive gore, a villain who's ultimately trying to teach his victims an important lesson, or an unpredictable twist ending).
Phantoms is also genuinely creepy in places, and keeps jump "scares" to a minimum.
Affleck and O'Toole are pretty good throughout their roles, the actress playing the younger sister does well with what shes given, and Schreiber is pretty effected at playing the creepy cop.
Incredibly, the pacing is pretty good here, unlike some monster films from the same time period.
The main disappointment I have with Phantoms is after being introduced as the supposed protagonists, the sisters don't get to contribute much during the finale.
Thankfully the creature's final form isn't really shown clearly making this monster feel like some indescribable monstrosity.