Faced them once or twice. Their claw+rend attack can almost 1-shot a PC if they are not careful. But their will save really sucks. May want to give them steadfast determination if you don't want the encounter to end with a deep slumber or slow spell.
Ugh, no, flashbacks! I had a Bbn3/Sorc3 or thereabouts get taken by surprise when he took on an "ogre" that tuned out to be a troll living among the ogre-tribe we were assaulting. The PC went from nearly full steam to deep negatives because of that bleeding claw/claw/rend. He also happened to have the only fire magic available at the time.
Trolls are horrible. Like the other posters say, I think they're best when the PCs aren't sure what they're up against, or to used with variant descriptions or forms. Reskin a troll as an ogre mutant and watch the PCs freak out (mine especially

) when it starts rending everything in sight and doesn't go down like
all the other ogres in the cave!
Trolls also make fine ingredients for weird arcane experiments. Their regen makes them attractive for crazy wizards to mix with other creatures. It's kind of fun when a party goes foraging and brings back a rabbit, only to have it "come alive" again, snap at the PCs, and go hopping off with a pilfered hunk of meat clenched in its jaws.
I also once ginned up a beast that was accidentally created by a covey of hags trying to combine their own essence with the beauty of a nymph and the ferocity of a troll. The resulting mishap (of course) imperfectly merged all the creatures together into an insane hunchbacked multi-faced monstrosity with too many joints and fingers. It rampaged, threw magic, cackled and wept simultaneously, and even intermittently tried to kill itself in the middle of combat. I built it to be pretty low-level, so it wasn't hard to defeat, but it provided a healthy dose of wtf while it lasted.