I have ACR 4.1 installed on Elements5 which works just fine for my Nikon D70. Does ACR 4.2 have any features beyond having more cameras available. I'm really asking if I should install 4.2 if 4.1 is OK?
In ACR 4.2 the sharpening algorithms (or was it the noise reduction algorithms?) were modified. Clarity was added to the check box list for creating presets. And additional cameras were added to the supported list. There may have been other minor changes that are undocumented. Generally speaking, it is usually a good idea to update ACR as long as it is supported by the software you are using.