So far most fun I have had with combination of dog, Morrigan and Alistair. The dialogues between Morrigan and Alistair are just priceless. For example:
Alistair: I have been thinking...
Morrigan: Now I know why it has taken so long
And many other comments they make about each other are just hilarious, sometimes it's just hard to decide which one I should side with when their arguments get so bad that I must take part in the discussion. Usually end up with Morrigan, as her replies are more witty, and My goal is to try to romance her.
The dog is so cute and funny on it's own way. The "conversations" it has with both Morrigan and Alistair are just priceless at times, as well as it's habit to "designate" different places (for example, the dog decided to do that inside a crowded inn.
As for the skills and abilities of these characters, well, both Alistair and Morrigan have their uses, Morrigan mostly as spellcaster and archer, and Alistair as my main meat shield. It's at times hard to actually keep Alistair alive, but I think the same problem would come with other melee characters as well. In most difficult fights me and Dog are usually the ones who stand left if not everybody.
I wouldn't actually want more NPCs to the game, as quantity doesn't always overcome quality. 9 joinable characters, each one well made with interesting personalities and skills, is better than 12 uninteresting characters, made hastily. And if we are precise, besides the normal 9 NPCs, the temporary characters are as well well made (the origin part NPCs aren't that well made, but interesting characters enough for their purpose). especially if we take account how long they will actually tag with you.