I've rated everything A Song of Ice and Fire five stars so far -- even A Feast for Crows! -- because I look at the series as one complete work, and each volume gives a lot of detail to that complete work. I understand that's how some folks look at Tolkien's work, too, so I don't see that as being an unprecedented position.
With these two "stories," though, that trend stops. I wanted to like them. I liked the Dunk and Egg stories, so I expected this one to follow in the same vein, but what I got was a couple of dry history lessons narrated by an old Maester. So much of the events in these stories were glossed over to the point of them reading like analytical pieces and less like stories. There were antagonists and protagonists (or as close as you can get to them in this series), and there were clear conflicts and resolutions, but the lack of the point-of-view characters killed what interest I should have had with these sto-- ... er, essays.
Taking the broad view, I can appreciate that they fill in more of the Targaryen's history, and find that a lot of the content here is reminiscent of what would normally be found in Westeros. Political machinations, deceptions, assassinations, battles, betrayals, and power struggles are just the surface of what you'll find in these recountings of the Dance of Dragons. In fact, what's in these pieces could easily be the outline for another novel set in Westeros without it being part of the series proper. I just wish that Martin had treated them as such.
I mean, anyone who's interested in the series and the expanded history beyond just A Song of Ice and Fire should read these stories. Just don't go expecting them to be anywhere close as interesting as the series proper.