i don't use it often, but its saved more party wipes than anything else.
its great for getting threw outdoor zones fast, or even getting across the city.
picking up soul stones doesn't dispell it. combine it with invis and stealth and you should be golden. yes you still can be seen unless you use stealth or invis, and even then if you run right in front of someones auto detect range... just dont fight back and you will stay Shadowed and keep your 40% displacement.
it takes some practice to get used to, just run around the city with it on a few times.
Is it worth the feat? don't know... i use it, the invisibility and displacement gets more use out of the dragonmarks i have. since you can cast them on other people.
but its been the source for many a great rescue missions.