Well I personally have not tried 8 or more strings, so I can't really comment on those too much. But that Skrillex cover was pretty cool. Very energetic playing.
I only stick to 6 and 7 strings so far. 7 can be quite cool I find, be it for extra big clean chords, adding a little something to existing riffs (intro riff to Cowboys from Hell you where you slide down from the high part to the low part...I can do that on the low part too) or just playing further with the range. To me it is not revolutionary, but being able to have an extended range can be quite cool. When I play the vers and chorus to "Down from the Sky" by Trivium most notes are played on the normal 6 string range, but some go lower, which really gives them an impact. Plus I also have to go up to high E over the 12th, so I am using the range.
The way a lot of people seem to play them is to me actually a overuse of the extra string, when they play it mainly. In that case a lot of them might just as well tune down a 6 string. I prefer to mainly play it as a 6 string, since that is very possible and comfortable and do some lower notes, which reserves the impact. Dunno if you see what I am getting at...
On the bass side things work easier a lot of times. Still many people who should rather just downtune, but cool guys out there too. One of my favs which I like is the guy from Unexpect. Weird music, yes but I like it. That guy plays a 9 string bass, not guitar and does a lot of tapping. Of course he also goes into guitar range, but he works with the guitar parts and delivers a different sound. This video a quite a good example. Guitar is not loud enough, but you get the idea. I think that utilisation sounds quite cool.
http://youtu.be/qLeXaxlSbc8
