None of the reviews reflect the real world behaviour of the lens. In some cases, it's been proven that they've swapped the samples around, probably by mistake: 'This image is sharper therefore it must be the Plus'. My side by side tests consistently showed the Black to be noticeably sharper every time.
It might be that the review version is different to the consumer one, in which case the reviews are largely pointless. Either way, the Plus will give you a softer (best described as 'a bit mushy'), image unless you have it pointed directly at your face for the entire sequence.
Something else to take into consideration. The Plus has new contrast and colour control profiles. These gives a very dynamic and punchy image if you leave Protune turned off. It achieves this at the loss of almost all detail in dark areas. The original Black's auto profile retains an excellent amount of information in dark areas, allowing for a good deal of tuning, even out of Protune. The Plus pretty much records a deep shadow as an untunable pure black mass.
Edit: Youtube is never a good way to judge image quality but the Bikeradar footage does seem to show the issue, especially in the forest sections in the second half. Running the raw footage on the PC really brings this out.