Is the FFXIII trilogy worth playing? I would say yes, it's worth it, but of course, that depends on how you feel about the game. I honestly think that the "linear" complaint doesn't make sense. What game ISN'T linear? You go from point A to point B to proceed in the main story, and you're limited by walls and boundaries. That describes just about every game ever made. FFXIII doesn't offer much as in exploration except for 1 or 2 comparatively larger areas, so in that sense, I can see why people would complain about XIII's "hallways."
I think that XIII shines in its CG cutscenes and character development. I grew attached to the characters more in the first game more than I did in the sequels.
FFXIII-2 had improved and faster combat, but it was lacking the character development (and CG cutscenes) that shone in the first game.
LR is the grand finale, so it's worth playing it for closure (if you liked the first 2 games). The graphics are definitely a step down from the previous games, but the gameplay is very different and a refreshing change for the last game.