I've read that the main story line is merely adequate but I've been so preoccupied with side missions that I really haven't even bothered too much.
This is really an impressive game that was not on my radar because of the mixed reviews it received. I guess reviewers just don't enjoy old school RPGs anymore.
I highly recommend giving this game a shot because it really gives you a feeling of exploration, sort of like the first time you fired up Legend of Zelda. There is always something new to see and new quests to do. If you want a more story centered game, Mass Effect this is not. If you enjoy leveling, looting, and exploring, this game will suck in hours and hours and days and days of your life.
The only negative I found is that the number of enemies can often turn the game into a slow slog through the hordes until you reach your next objective. That being said, you can always purchase a horse or unique character class mount and just run around the hordes to get to your next quest. This is a small inconvenience in an otherwise fantastic fantasy setting.