Most obvious is better performance. Everything the app does is now noticeably faster, which makes it all the more fun to use.
But there’s also some new edit functions; iPhoto-style head-up panels for adjusting images, and better still adjusting their geometric placement within the stitched final result. With the geometry controls, you can rescale, tilt and rotate images in situ; this is a huge help when it comes to fine-tuning the stitching of one image to another.
What’s a little confusing is that there’s plain old version 2.0, but also a newly-released version 2.0b17 - I’ve managed to get both of them side-by-side in my Applications folder without intending to. The beta has some separate release notes which detail even more new stuff; the Free Perspective mode looks particularly fun, and there’s also Exposure matching and QTVR export too. Lots of stuff to play with.
Doubletake 2.0 is a free upgrade to existing license holders; a fresh license is only $16. The new version requires OS X 10.4.