Depends on what you like.
DA:O was a brilliant game, the best Bioware game for my tastes, and the last great tactical RPG they ever made (ME1 was legit too).
After that they made ME series into a plastic shooter with RPG elements, ruining the best space opera setting I've ever seen, and they tried to transition DA into an action-RPG but were utterly crap at it.
Like NyQuil said, DA:O has great tactical combat, is fairly large, has a decent amount of choices that you can make and effect the world, the story is excellent with the overarching plot being just the grounds for the "real" story which is about human conflict and morally grey characters, companions weren't horribly simplified and stereotyped like in later DA sequels, etc.
It felt like an old-school isometric RPG (and you could play it like that) with some more modern elements.
Demons Souls is what started the Dark Souls series, and its brilliant, but rough round the edges.
If you were looking for a Souls-like experience, IMO just pick up Dark Souls.
It's hard and punishing - but fair - until you learn the mechanics, then it just becomes brilliant.
KOA:R plays like a single-player MMO.
Its fun, good classes, good combat, decent story (RA Salvatore was working on it lol, dude who wrote so many Drizzt novels, and I gulped that stuff up when I was younger, pulp or not pulp - and it WAS pulp
).
I played through it twice, and I had to delay ending it both times as I got burned out on same-y quests etc.