I personally use Ultramon at work and home and love it. However, all I wanted is to have the taskbar span my screens and I didn't want anything too bloated. The one feature it's missing though is assigning programs to specific screens. Sometimes prompts from one screen will come up on another screen... which is annoying, but I deal with it.
I did try out Display Fusion when Scott Hanselman mentioned it on his blog. At first I really enjoyed it because it did allow you to assign programs to monitors, etc. It gave much more control and the start menu icon was on every screen. After a few days though it got kind of buggy (mainly the task bar became unresponsive). So I ditched it even though it did everything else better. They are coming out with a new version, 4.0, which I do plan to try out once it's out of beta.
Both Ultramon & DisplayFusion have a 30-day trial. After 30 days, you'll either need to upgrade Ultramon or just continue using the free version of DisplayFusion.
I would try them both out (but make sure only one is running at a time) and see what works best for your setup.
I'm running Windows 7 Professional with 8GB of Ram and Xeon W3530 2.8 processor.