I just want to point out the math behind SoSa. If your opponent is dealing you X damage per turn, then SoSa is the equivalent of 4(X - 10) points of healing. For example, if the opponent is dealing 20 dmg/turn, SoSa is worth 40 points of healing to you.
The calculation is simple: without SoSa, over 2 turns the opponent deals you 2X damage. With SoSa, you take 40 damage initially, then regain 2X health, for a total of 40 - 2X damage. So the total damage reduction is 2X - (40 - 2X) = 4X - 40 = 4(X - 10).
SoSa has 3 additional factors that affect it, beyond the simple 4(X - 10) figure:
1. SoSa only works if you have >40 HP
2. The opponent might be able to temporarily reduce or postpone the damage he deals while SoSa is active, reducing the effective healing
3. The opponent can't dramatically increase the damage he deals while SoSa is active, so in addition to healing, SoSa buys you turns where you are immune to surprises.