I do not own one myself, however there are plenty of comments around the web about the MC4. There are a plethora of comments about strong vignetting on the MC4, especially on FF but even when used on APS-C. IQ is better in the center, but commentary also seems to indicate that IQ is still not generally as good as the Canon 1.4x TC III (despite what the german article indicates).
I got a 300 DGX Blue Dot, however if the MC4 had been available for purchase at the time I purchased, I'd probably have one of those, despite the vignetting. At the time I bought, it simply was not in stock at any of the major distributors (Amazon, Adorama, B&H...I generally don't like buying from anywhere else, had too many problems.) I waited for a while, and eventually got the 300 DGX.
From what I've read, I am honestly not sure that the core IQ issues that I experience with the DGX are fixed with the MC4. My primary issue with the Kenko I own is not really with vignetting or corner sharpness...it's with the way it convolves detail, and especially the way it renders out of focus blur. The MC4 might be a little better than the 300 DGX in the center for the focused target, but the "scratchy" background blur, the warped and spotty highlight blur circles, and "rough" detail (it can be sharp...but there is something else going on with it that makes images produced with the 300 DGX just not as appealing as those produced with the 1.4x TC III), as far as I can tell based on commentary and sample images those issues exist with the MC4 as well (and probably most/all other Kenko TCs).
If you don't really care about your background blur, the MC4 should be sharp enough, but it still doesn't seem to give the same kind of aesthetics to it's IQ as the Canon 1.4x TC III does.