I powered through the Historical battles (albeit on easy) and completed the Galactic Conquest with the CSI which was great fun, especially on Kamino and bespin.
Geonosis is a nasty level if you stray to far from using the republic tanks, life for a foot soldier isn't the most practical with those CSI wheelie machines rolling around.
I'm still buzzing . . .MWHAHA, the CSI are Godly, droidekas, super battle droids . . . MTT's, STAPs! It never ends!!
Random Off Topic Halo 2 Tidbit: Rumors circulating that split screen is available for XBox Live. In other words, you can play with your friends on Live...on the same XBox. Hasn't been confirmed yet, I believe.