I have used both Sidetrack and iScroll. I finally decided to use iScroll2. Sidetrack has a lot of nice features. You can hot corner the trackpad, set "touch and click" to different mouse buttons, and vertical/horizontal scroll along the sides. But it cost $15. While iScroll2 doesn't have as many features, it does allow you to scroll horizontally and vertically and the "touch and click" feature to the trackpad. Finally, iScroll2 is free. That is the main reason I use iScroll2 over Sidetrack.
Note: for those who don't know what I am talking about when I say "tough and click," these programs allow you to configure your the trackpad to when you hold you finger on the trackpad and click the trackpad button, a different button is registered depending on your settings (I had mine set up for right-click).
I really like iScroll2. I've come across some trouble when trying to mess with all the options, but if I leave it on default, it works perfectly fine.
I've actually come to realize that I can no longer live without trackpad scrolling, and I think it's great that there's an option for us people with PowerBooks and iBooks bought pre-trackpad scrolling.
Also, I can actually make my Windows friends jealous with trackpad scrolling, as many of them have laptops that didn't come with the ability to scroll and they can't find any programs to make enable any trackpad scrolling.
And FistfulofAngst mentioned the best part: it's free!
I wish I could use iScroll on my Tibook 400. Sadly enough, the installer said it wouldn't work, so I'm using Sidetrack, and I keep hitting the "register later" button. I don't use the Tibook often enough for the reminder to bother me too much.