The first thing I noticed is that the movement is much, much smoother. The ThinkGeek Megaminx locked up incessantly, but this one jams quite a bit less. It still is an issue, but it is much better in this version.
Another gripe I had about this puzzle is the color choice. Generally, it is not bad, but for some reason they chose to have two shades of orange on adjacent faces, which is quite confusing. I found myself needing to move to an area with better light so I could tell the difference. There is also a blue and teal that are quite easy to confuse. The ThinkGeek version also had this issue, but at least similar colors were not adjacent. I guess it is hard to choose 12 colors that you can't confuse!
There were also a number of spots where the stickers were pretty far off center. Not a big deal, generally, but some are close to the edges which makes me worried that they'll get caught and peel off.
With all the gripes I'm pointing out, you'd think that I didn't like this puzzle! Despite all of this, I'd definitely recommend getting a copy: it is about $30 while the ThinkGeek Megaminx is $10, but the much smoother operation is defnitely worth it.
If you haven't tried a Megaminx, this is definitely a good twisty puzzle to try if you want something significantly more difficult than the Pyraminx. I think most people will be able to solve a good amount of it by thinking their way through it, but the last layer is difficult. If you get stuck, there is a great solution algorithm posted here.