What are the advantages and disadvantages to jailbreaking an iPad?
For


You can run any app you like, including ones not permitted by Apple, and you can run them at a system level rather than the unprivileged level they do on a normal device. 
You can remove Apple services on the iPad more or less at will. 
Provided you can download the image of the software, it's relatively easy to restore to a non-jailbroken state. 


Against


Running apps at system level comes with inherent risks - malicious code can kill your iPad, and badly written code can cause an entire system crash / instability. 
It's my experience that jailbroken apps (Springboard ones particularly) deplete the performance of the device (But I'm talking about an iPhone 4, rather than the more powerful iPad 2 - may be less valid). 
Jailbroken iPads aren't elegible for warrantee - in theory if you reset back to default you're okay, but if that's not possible you've invalidated yourself and they are totally within their rights to refuse you service. 
Jailbreaking needs to be redone with every software iteration, and with it, so do your jailbroken apps [possibly no longer true, can anyone verify?]