Paris is mainly flat. There are four tunnels that you run through and the exits are the only steepish bits that you will encounter and they're not that bad.
There are a couple of sections overs cobbles but don't mistake these cobbles for proper cobbles. Just to give you an idea, the start of the run is along the Champs Élysées which is a very busy road for traffic - it is cobbled. If you didn't know, you wouldn't really notice.
RD2 - I ran Berlin last year with some others off this thread. It was a good marathon with some great support and interesting sights but it also takes you through some areas that weren't that attractive.
However, you can't beat running through the Brandenberg gate.
If you get the chance, watch some of the inline skating marathon on Saturday, it's quite a spectacle.