I just finished Saira today after buying it on Steam the day it was released, and I am in the exact same boat as the Original Poster.
Knytt Stories was my favorite Nifflas game so far, and so when I saw Saira on Steam, I bought it without even giving it a second thought or trying the demo.
Following is my personal review of the game:
Pros:
1) Overall, I enjoyed the game a lot and felt it well worth the $8 I paid for it (I think Knytt Stories was worth money too [if it did cost money]).
2) The graphics are simply beautiful as well as the envirnoments and background, all much improved compared to Knytt Stories. I also love the character design of Saira (even down to the details of when she grabs her shoe [idle animation]) and consider it the best character so far out of all the Nifflas games I have played.
Cons:
1) As with some other posts on this forums, the game is a TAD bit too hard at times, mostly with the platforming and puzzles. Both the platforming jumping and puzzles were far more difficult than compared to previous games, e.g. Knytt Stories. Although the game certainly isn't impossible, Steam reports I have 5.4 hours on the game, so it can be beat in a fairly short time. Still, I think if it had the same moderation as Knytt Stories (just right), then it would be better (or at least future games).
2) With the original Knytt, the game seems to all be about ambience and exploring, which is what was great about the game. However, due to Saira's focus on difficult platforming and puzzles, you can't enjoy the beautiful envirnoments as much as you could as compared to Knytt. Ideally though, the balance in Knytt stories was the best, where you still add some goal orientation with the power-ups (which were really great power-ups [I really missed the umbrella]) and yet can still enjoy the envirnoments.
3) The music, although very good compared to many other games, was slightly less impressive compared to the music in Knytt Stories.
In summary:
Saira is a great game and well worth the money.
It probably is obvious though. I have a bias towards Knytt Stories in my review.
Thus, my final summary concludes with this:
Saira graphics AND Saira's Character design + Knytt Stories Gameplay AND Music = perfect game
All in all, I thank Nifflas for making great games and look forward to enjoying and playing more Nifflas games in the future!