Yes they are compatible and simply uninstalling the webrep as Jack 1000 suggests is possibly the worst suggestion you could follow.
If every one uninstalls it how is it to mature growing the database to make the results more relevant, so people should use it to rate the sites that they visit. Not to mention when the other proposed developments are introduced (data based on the avast virus labs info, etc.) it will be more of a useful feature.
This is a good point and yes, it would help Web Rep develop by having it installed. But my concern would be, if WOT says a site is bad or good, and Web Rep says the opposite, I think this may cause more confusion for the average user than, "double protection" from both plug ins. However, this is just my opinion.
Note that it is still interesting to study statistical views of sites comparing how WOT and Web Rep evaluate them. David is correct that Web Rep needs a chance to mature with and within the Avast community.
It will be interesting to see how Avast will use virus lab data to correspond to Web Rep site ratings.
Jack