Couple things. One, the spell works very uniquely on PTR. Think of it like a dot that you apply after having 3 dots up and that SW;I is not a channel. I mean that you can cast other things in place of it. For example, you can cast MB while channeling it and not interrupt it. See Here, I am Mind blasting while the channel dmg is going:
http://i.imgur.com/Xtvnw.jpg
Two, is that it seems to scale much better with SP than mind flay, just a higher scaling coefficient. However, right now it seems to not be scaling with mastery (possible PTR bug), or haste (less scaling with lust/hero sadface). It has a hard 3 second cast timer, if you call it that. You can also "cheese" it like with a haste proc MF. You can MF;I right as DP is falling off and get a full 3 second MF;I cast. Three is that, with this ability it might be better to cast only 1 Orb DP and keep the buff up. There are two reasons why this might become superior. The GCD gain from a 3 orb wont outweigh a MF;I and with them making DPs dmg over time static instead of the large upfront dmg. This will also increase the rating of mastery since a DP tick will be a much larger dps increase from it's current form. I have a strong feeling that crit and mastery will greatly outweigh haste with this next tier because of the set bonuses. The value of haste will greatly depreciate because of the set bonuses and it will make crit and mastery valued more. More sw;P crits = more SAs and longer duration on our dots, which will lead to more mastery ticks from a higher valued dot ticks and SAs. Just well scaling rotation now. This of course is just me saying what I think and no proven numbers yet. With the set bonuses and the change to SW;I, it's very possible that shadow will become a power house spec.