I play a Ranger in a fairly low-magic setting, where the PCs are engaged in a long journey through harsh and lonely mountains seeking aid for their home town.
He's a two-weapon wielder not through choice but because, by the time I got to creating my character, other niches had been filled. Out of a party of seven we had only one all-out melee fighter, and an elven fighter-mage (we started at level 4). This was a bad choice for the character - he had only a 10 Constitution (the DM insists on rolling in order for abilties).
My experience? That at early levels he really sucked in combat, though a goodly part of that was due to his low hit points (I take average since I can't afford a low roll). The Fighter always achieved far more than I did in combat, and that was with the benefit of me having Cosmopolitan, giving me access to Tumble as a class skill. I could get into a fight only needing to retreat almost straight away if something big got a hit on me.
But... since I turned level 8 and got a hold of a +1 and a masterwork shortsword (dual short sword wielder, btw), I seem to have become much better. I can now tumble past opponents to strike at mages and clerics, or to get flanking at the rear. I can take a couple of hits and not have to flee now, so I can get some full attacks in. Four attacks that often seem to land 2 to 3 hits is good, even when each individual hit is poor. And when I go against my favoured enemies (humans and undead) I get to be really nasty. Most recently I chopped a powerful ghast to pieces by myself, and forced a cleric who was acting in support to flee for his life (only to get run over by a charging celestial bison!).
So, I am beginning to think that whilst TWF is weak, with the right choices of feats and tactics, a TWF-Ranger can be lethal. But it very much depends on getting reasonable kit and knowing your enemies.
Well, I'll get another chance to test this. I start running a new campaign soon, and one PC is a TWF-Ranger (1/2-Elf). We've already talked character design - next we'll talk tactics. But I have high hopes for him in conjunction with the party Druid and animal companion(s).