I'm using RRS L-Plates with a 1DII and a 1DsIII. I also read articles about the slight softness in vertical shooting position. I tested and found that, at least for my setup, there was no softness. I got identical results in vertical or horizontal orientation.
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I'm using RRS L brackets on my Canon 1D2, 1DS2 and G10. Also no evidence of any image softness in portrait orientation. The fact that the L bracket keeps the camera's mass centered over the tripod and not off-balanced to the side, plus I do not have to re-adjust the height of the tripod are huge advantages.
In the RRS L bracket you have to slightly re-orient the L bracket to access the cable release port. Not a big deal IMO. However, I rarely use a cable release opting to use mirror lockup plus 2 second self-timer.