I finally had a chance to log on and check out the new Spirit Shell, and it's very interesting. There are a couple of interesting notes about the way in which it works.
Casting Spirit Shell and then multiple different healing spells stacks up different "shields", each of which is named after the individual healing spell. For example, the following two screenshots show what happens when you cast Spirit Shell followed by Flash Heal and Greater Heal.
Casting Spirit Shell and then the same healing spell stacks up different "shields", each of which is named after the individual healing spell, although there does appear to be a limit to the stacking, as you can see in the combat log:
Interestingly, it appears as though the stacking cap is per each healing spell, so in some cases you might be better off casting multiple, different healing spells. However, in practice, your targets will likely be taking damage, and the shields might not even have a chance to stack up to their maximum value anyway.
It's still early, but Spirit Shell seems like a very interesting spell, although it is certainly a significant departure from the original goal of Spirit Shell, I can see it having a few very interesting uses.
All in all, I like what I see so far, and Spirit Shell should prove to be very useful.