Personally I thought Demandred was spectacular and well captured by Brandon Sanderson. People have been very cynical regarding his hidden identity and that nothing could make up for his revelation no matter who he was. I don't really agree. I felt that way prior to reading AMOL and when he was revealed he was magnificent.
As a character, I thought he was one of the most imposing of all of the Forsaken to date. I loved the way he spoke and enjoyed reading everything he had to say, especially how he dripped knowledge from the AOL and how he explained his feelings on so many issues. As expected he appeared in a fantastic suit of armour on a white steed, and was worshiped by his army as some sort of saviour/demigod.
He was set up as one of the great generals of the Shadow and didn't fail to disappoint. Only Mat could really match him, though Demandred did owe a portion of his success to the catastrophe that Graendal caused among the great captains. Brandon reflected his intelligence and cunning well. I felt you could really see why he was one of the greatest men from the AOL - I often felt that there was nothing that could defeat him, and that he could adapt to anything.
In a similar vane, I found him hugely impressive as a channeler - far more than I thought he would be after his mediocre performance at the Cleansing. Demandred must have personally killed a hundred thousand people in that battle. I loved how he utterly humiliated (and almost killed) Taim using complex weaves of the True Power, with an almost indifferent attitude and a marvelous speech while he was doing it. His duel with Logain was similarly impressive, using weaves that Logain couldn't understand or penetrate. It was fantastic. I enjoyed small things, like how he meditated to fend off the exhaustion from channeling, how he could bind a sa'angreal to him, pulling Aes Sedai out of the darkness, calling his enemies 'little people' all the time etc...
It was also good to see his skill with the sword, and how no one could surpass his skill. His duel with Gawyn was excellent because he killed the idiot, and that is a worthy accomplishment. Even the Aes Sedai were glad that he died. His battle with Galad was more impressive and I liked their banter. Brandon built Demandred up so much, that I didn't feel let down that Lan finally killed him, especially because it was plausible. Lan has always been the greatest swordsman in WoT, he's cool, he has a ter'angreal, and most importantly he went after Demandred so aggressively that he couldn't channel without dying. Lan also had to sacrifice himself to defeat Demandred.
I liked how Brandon portrayed Demandred's insanity and obsession. He was so hateful and focussed on LTT that he could barely think of anything else. Many characters remarked on it. The man was truly disabled and limited by his hatred. Rand, meanwhile, never thought about Demandred after Merrilor (even Moridin irritated him). We got a glimpse as to what a great man he would have been had he not turned i.e. leader, comrade, lover, but it was all smothered by this one singular goal that he blurted out across the battlefield with saidin. Even Mat thought he was insane.
Lastly, I enjoyed how Moghedien bounced up to replace him. She's always been a source of amusement. From when she burned his body, to her illusion to imitate him (she thus sent the 100,000 trollocs btw), was excellent - until a legion of canons fired at her through a gateway of course. Her facial expression must have been priceless, I hope the Ash'aman took a picture.
Overall, I'm not sure if it was a mistake leaving Demandred until the last book. We did see him in book 6 and 9, so we can't complain too much and he was a virulent villain in AMOL (alongside Graendal - the woman is a demon). I'd give him a solid 9/10, and I only rate Graendal above him overall in the series.
Fan of Lanfear