Very nice and very nicely done. I like that little bit at the end. = )

I don't know why but now I want to see flour sack adventures in which he tries to escape a baker that wants to turn him into muffins.

Nice job. The only thing I can say is that the landing is a bit sudden. Perhaps slip in a few frames of the sack squashing a bit more. You could have the bottom squash down with the impact while the top half stretches forward (stretching a bit). Kinda like taking the initial jump and reversing it with a few minor edits to show more squash (from impact) as opposed to stretch (from leaping). Everything else looks great though. Especially the frightened jolt when it looks down, haha.
Hope that helps (or makes any sense, haha).

waaahh It's so great to see some of your animation tests!
I know you pposted this awhile ago, but commentary wouldn't hurt even if it's over a month too late.

I love that you put character into the sack not only before the jump, but after too! it's that extra bit of time and effort that makes it better than just watching and practicing a simple jump. I do agree with mogbox though, the ending did feel a bit abrupt, like the jump was too quick. Perhaps also with more squash, you could have that moment of delay that relatively heavy objects have before they drop completely (the highest point and stuff, you know what i'm saying i don't need to be specific and wordy)

very nice!! 2 small critiques

1. more time on the pre-jump. have it curl down with preparation for what its gonna do. extend that 3 frames to about 5. give it like... an inhale before it bends it's 'knees' to jump.
2. have it look two ways in the first part. i couldn't tell quite what direction it was facing since the perspective is very flat.

marvelous job!