First, they are the backstory. Before we turned to immortal spider-kings, nazis cum other fascists, and interdimensional duplicates, the original interdimensional attackers were the Rikti. The whole story behind the City is that the Rikti attacked. I like getting back to that story.
Second, there are some good Rikti stories. The Dark Watcher story arc (“The Horror of War”) is excellent, one of the best in the game. “Division: Line” and “The Organ Grinders” are fundamental arcs in getting the CoH backstory. Lady Grey’s Task Force does not have a great story, but it has interesting ideas and a great twist at the end.
Third, in terms of gameplay, they bring a lot to the table. There is something there for everyone, which is to say that there is something hard there for everyone. Their drones see through stealth. They have every type of crowd control, which is rough on squishies. They have lots of psychic damage, which is hard on most non-squishies. They have hordes of little monkeys to fight. They summon portals that are worth ridiculous amounts of experience. They have Guardians, who will give you nice buffs if you land a confusion power. The ideal anti-Rikti PC is probably a Spines/Dark Armor Scrapper: status protection, psychic resistance, PBAE damage toggle for the monkeys, and a single-target ranged attack to aggro without disrupting the portal summoning. That could explain part of why it is the flavor of the month.