Update. This DLL can be referenced , but there is no way to subclass it. Even if you try to set properties thru reflection in code it will not work. This is something that MSFT has done on purpose and I don't know why. For now, I am faced with writing my own using the reportviewer control. There are a few things that are involved in implementing (user specific parameters and setting those). I have found some issues with this webpart when being used in integrated mode with sharepoint 2010 - for instance , if you are using trusted account for the integration (as suggested by MSFT) and you have multiple of these webparts on a page, you will find that any subreports on any of those reports will fail to execute if launched from the report or a link on that report. MSFT tech support is currently looking at workarounds for this behavior - I thought I'd share this in case you have to implement this.
Multilingual support on RDL's is also an issue. Even if you set the Report language to =User!Language on a report and your browser setting is set to something other than english (and you have the right language packs in place in sharepoint 2010) you will notice that the Locale stays at US-en when passing to a report in integrated mode. I will update this if I find a workaround.