With regards to speed – When I had winterboard installed (Think Themes for iphone), things did run slower. Also, I have heard that applications that allow programs to run in the background (one such application is "Backgrounder", you can take a speed hit. Also, it is possible to run out of system usable memory if you put too many gadgets in. Read up on any apps that you think you would like (Google search, or here), or ask if there's a specific application. As far as should you? I believe that there is a standard "disclaimer" on jailbreaking programs. But too many people do it for it to be too dangerous. "Insert standard disclaimer here".
And again, if you updated to 3.1, you will have to wait until the new version comes out. It will probably be posted here.

It actually isn't that hard if you follow the instructions. If you've already installed 3.1, you need to wait. When I jailbroke, the hardest part was getting the device into DFU mode (Device Firmware Upgrade). Timing issues. But lack of functionality of the platform (Appleitarianism) is the reason why people jailbreak. Something as simple as letting me install my own SMS tones. Turn wifi on and off without having to go thru the settings (SBSettings). Sooner or later, the Warden will have to work with his charges. Or people will keep trying to escape.