I'm still using LR3 with DNG converter from adobe for processing my 5D3 files. The reason is because all of the reviews I've, the biggest compliant is the processing speed in LR4.1 seems to slow down a lot over time at least for the windows version. So I'm staying away from it for now until they have it fixed. The reviews on Amazon says that even the Adobe developers acknowledge that this issue exist, but doesn't seem to have a fix for it.
Anybody running LR4 on windows have the slow down issues?
Yes and no. If I'm doing a lot of cloning/healing it will start to slow down, but this is partly because of how LR handles these functions compared to PS. Also, if you do lens corrections first it can slow things down a bit. Another area is TIFF files after they come back from PS. I personally think this has to do with how PS compresses the TIFF when done as a Save, since I don't have this issue with TIFF files generated for/from plug-ins like Photomatix or even Perfect Resize. Of course, I'm running an AMD 1090T 6 core processor and 16GB of RAM which I'm sure helps things out a bit.
Once I downloaded LR4 I could have literally kicked myself for waiting.