The eShop reviews are a terrible way to gauge sales because the eShop never reminds people about submitting them and the option to do so is not so obvious. But Pushmo already has more reviews than top-sellers like Plants vs Zombies.
It may very well be a terrible way to gauge sales from a numerical standpoint, but in terms of comparing the games to each other, it actually does a pretty good job.
I actually track monthly ratings numbers on the US eShop (because I'm obsessive over numbers and all that):
http://thirdrateminion.blogspot.com/2011/12/eshop-sales-ratings-november-2011.html
The Ratings Top 20, November:
1 The Legend of Zelda: Four Swords Anniversary Edition
2 FreakyForms: Your Creations, Alive!
3 Super Mario Land 2: 6 Golden Coins
4 Legend of Zelda: Link's Awakening DX
5 Plants vs. Zombies
6 3D Classics: Excitebike
7 3D Classics: Kirby's Adventure
8 Super Mario Land
9 Photo Dojo
10 Inchworm Animation
11 Kirby's Dream Land
12 Mario vs. Donkey Kong: Minis March Again!
13 Bloons TD
14 Cave Story
15 Let's Golf! 3D
16 Donkey Kong
17 Zenonia
18 Mario Clock
19 Anonymous Notes Chapter 1 - From the Abyss
20 GO Series: 10 Second Run
Nintendo's Official Top 20, November:
1 The Legend of Zelda: Four Swords Anniversary Edition
2 FreakyForms: Your Creations, Alive!
3 3D Classics: Kirby's Adventure
4 Super Mario Land 2: 6 Golden Coins
5 Legend of Zelda: Link's Awakening DX
6 Thriller
7 Plants vs. Zombies
8 Super Mario Land
9 Kirby's Dream Land
10 Photo Dojo
11 Bloons TD
12 Mario vs. Donkey Kong: Minis March Again!
13 Night of the Living Carrots
14 Pyramids
15 GO Series: 10 Second Run
16 Cut the Rope
17 Let's Golf! 3D
18 Inchworm Animation
19 Mario Clock
20 Nintendo Countdown Calender
(and most of those here which were not in the Ratings Top 20 were <10 outside it)
There are some discrepancies, yes, particularly by the 3D videos' performance. Still, the monthly ratings are decent barometers of sales, considering we get no other information other than developer commentary. Unfortunately, given some ratings numbers, this does not bode well for some DSiWare devs :/
Considering Freaky Forms and Let's Golf 3D are close to 2000 after being online for a month and about five months, respectively, for Pushmo to already have 500+ ratings in less than a week is a good sign for its sales, comparatively of course.
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Back on topic, Pushmo has been messing with my life. I keep wanting to find more ingenious puzzles. I'm over half-way done with the main game, but if people keep making more Pushmos, I don't think this will end any time soon <3
I'll get to posting some Pushmo soon!