Everything that's new and changed in After Effects CS5 is described here. See the comments on that page for some reviews and such.
Here's my personal take.
Every change from After Effects CS3 to After Effects CS4 is here.
You asked about motion tracking. The inclusion of mocha-AE makes for a huge change.
Yes, absolutely. This is the biggest upgrade since the move from AE 7, from what I can tell.
Just make sure you have a system that can make use of all the new features. Get lots and lots of RAM.
Aside from new features CS5 offers, one of the larger boasts of this upgrade is in its performance and memory.
How much faster is CS5?
Any actual users out there can testify to this?.. if possible can you please provide an example of a speed comparison from a project in CS5 to an earlier version?
From CS3 you will be totally blown away, so, yes. If you were upgrading from CS4, the changes in Todd's list appear attractive if you really need some of them. Otherwise, there is a large constituency of CS4 users who see the new features as incidental, incremental, and too focused on niche users. As one of about 200 Adobe seats in a corporation I will not upgrade for many months, not until the whole otufit moves forward. I seem to be surviving.
If you work in HD (or plan to),
If you have more than 4gb of Ram,,
If you have a multi core proc
And if you have a 64bit OS, then the speed jump in render time between CS3 and CS5 is a huge blast. I don't have numbers on me, but I had to switch back to CS3 for an older project, I hope I'll never have to do this again. Not only that, but it's nearly the end of the infamous "Cannot allocate buffer".
Plus all the smaller enhancements like separated xyz, better camer control, etc etc, you should defenitly move to CS5.