Though I liked The Witcher, it is not without its problems, many of which have been mentioned here. Still, comparing it to DA:O is tricky. The Witcher has no party system and features a limited supporting cast, many of whom are pretty interesting, but not intimates of the main character. The choice/dialog system impressed me a good bit as well, with its rock-solid commitment to gray moral areas and difficult decisions, but is constricted by the main character's existing personality. It is also far less open and, to a degree, less polished than DA:O.
What it comes down to, basically, is what you enjoy in a game. In order to enjoy The Witcher, I feel like you'd have to prioritize story and complexity above gameplay, atmosphere, and (to some extent, at some points in the game) characterization.
Short version: I liked it more than enough to justify a $20 price tag, but it is an odd beast with a small number of nonetheless significant issues. Hope that's helpful!