I'm a big Descent fan, and I was ecstatic to learn of the Trollfens. I love that it uses a swampy environment rather than the caves and wastelands that the other expansions and base game use. As a mostly overlord player, there's tons of new tricks with the Infector class, as well as the new monsters Harpy and Plague Worm. The Harpies have grown to be one of my top favorite monster groups of the series with their high speed and Swarm ability. Plague Worms are also incredibly useful with high defense and the great Burrow ability. As for new heroes, I haven't seen Roganna or Grisom in action yet so I can't really comment on them. But the new Prophet class is very popular in my group of heroes, and the Stalker class has a few fans as well. Bol'Goreth is a bit uninspiring as a lieutenant, however (especially compared to Valyndra the Wyrm Queen).
My gaming group is a bit impatient with games, so I'm very happy they included a mini-campaign for The Trollfens. The quests themselves are great fun, with my favorite of them forcing the heroes to save a wagon full of survivors before it sinks to the bottom of the swamp and they are killed. I definitely recommend getting this expansion over Lair of the Wyrm or even Labyrinth of Ruin, though ultimately I think all the expansions should be purchased for any Descent fan.