I split all the time. If you live in Los Angeles you can get very adept to it if you are willing. The traffic here absolutely stinks. During rush hours if you are on the wrong freeway you can easily be lane splitting for well over a 20+mile stretch before things break up a bit.
To me the danger lies in the speed differential that you are splitting at. Visually there isn't much difference if you are going (all hypothetical of course) 15mph with traffic at 10mph...vs. 65mph with traffic at 60mph.
The way traffic runs over here you'll go through miles of slow moving traffic, with small pockets gradually (almost imperceptibly) moving faster/slower...but the cars still stay pretty tightly packed. IMHO the tighter the pack the better it is for the lane splitter because it gives cars less maneuverability options.
Lets say you are travelling 10mph faster than the flow of traffic. As the flow speeds up and slows down you too are maintaining that set differential. Pretty soon you will be splitting at a relaxed highway speed and everything will still appear visually static.
Now if there isn't any traffic, it doesn't make much sense to needlessly split. However, for me, splitting at speed is an option to get out of "an accident could happen here" situation...and I will excercise that option without a second thought if I deem it to be a prudent one at the time. It can also be a much smoother, more mind-relaxing method than large, "erratic" lane changes, coupled with sudden bursts of acceleration/decceleration. Ughs.
Smooth and flowing is always the key. There should be NOTHING "wild/hair-raising" about it. And in the end, you balance the equation with "the patient rider lives longer..."
Sitting in the cage, with a restricted point-of-view visibility...lane splitting at speed looks completely insane.
However, when you are on the bike with tons of sight lines to choose from...things look completely different.
I will say that I often come up on other motorcyclists lane splitting...and you can tell which ones are very unconfortable with the idea because their lane splitting is very slow/erratic/with jagged lines=no good. And every now and then you will have the one that is travelling at speeds that exceed their ability to stop if need be=no good.
I once saw an excellent lane splitter. It was an older gentlemen on an old standard. He made very good time and just flowed through the traffic as if he was not even there...adjusting lane changes with lane splitting...all the while with good effectiveness/precision/control. Smooth and flowing.
Bottom line: If you are lane splitting in a manner which makes cars react to your presence...that is no good. However, if you lane split in a smooth manner so that it is like you do not exist to the cars (generally speaking)...THAT is good.
If you are going to do it, do it right...there should be no tentative "half-assing" your way through it. Set your line, set your speed...watch out for openings in traffic (those are danger signs) and adjust your speed accordingly. If another lane looks better to split, take it.