I enjoyed it, it did exactly what it advertised - let you play out some combat from the idea of your PC never having survived the joining. All your old allies are now level bosses effectively, which gives you some friendly old faces to slap around.
Things I liked:
1. When an ally dies, they are dead. Since you can give 'gifts' to your allies, giving them bonuses to stats and the more killing you do the more they respect you (thus their bonuses go up) this has an impact. It really requires you to manage your party intelligently.
2. You have to manage your non-party allies - protect the ones that you may want to recruit. Find a couple of ogres? Make sure they BOTH live, yours may get snuffed by one of your Origins party members up ahead and he'll be your pinch hitter!
3. Your abilities are not static, you pick up extra stuff for completing side quests. Also you do find 'loot', additional weapons, armor, gifts and such to either use yourself or use on your allies.
4. Brutal and hardcore as you'd expect of leading a Darkspawn vanguard into Denerim. Killing civillians, breaking up defenses, burning stuff down.
5. How the recruit/dismiss aspect works - again, it adds to the importance of minding who your allies are and what you do with them as well as what you give them.
6. Codexes, what they added to the game lore and such.
What I didn't like:
1. Wanted more interaction in killing old party members. Cutscenes, death cries, something.
2. Options for the Vanguard - better weapons/armor to loot, things that had a proper Darkspawn feel. Gifts left by the Archdemon? Empowerment of existing gear? Something.
3. No really dramatic moments before the end. It was ruthless, yes, but no dramatic moments to really drive it home.
I'd go with 7/10. It's a one-off. If you're playing it for social interactions, making friends and enemies, romances and doing the hero thing this isn't for you. It's a one-off, a chance to relax and play the game like a game and see DA from the eyes of the Darkspawn. No, that's not generally a rich and emotional experience - these are not the social sort from Awakening. These are the Blight, the ruthless, kill all living things they come across sort of nasties. It's $5 of chaos in a familiar place. If that's fun for you, this is an excellent choice.