I actually have a load of old Promarkers knocking about the place somewhere (not used for ponies, sadly. I don't have the bright colours to do it) and I was just wondering how they all compare. Have you tried many others for comparison?
yes I have tried other makes. or one. my friend got some from china or japan over the internet (whatever that is ^^) the colours are good, but the colour bleeds outwards from the nib more easily and it is almost impossible to get anything aproaching a smooth finish.
pro markers like I used, well, look at it. not super smooth, but pretty good. besides, there the kind my grandad used to use. also you can use the same colour again ontop to do the shading. again see this pic. (though shading is more obvious in real world)