I just got this today (before the upcomming nerf in patch 2.1.0) and tested it out a little. When you use it you get a debuff saying you get 150 health every 3 seconds for 15 seconds.
That's all well and good, however, when I shift into Tree of Life it ticks for 261 health. I have 446 spirit at this time and 25% of that is about 111. So it turns out that the healing boost from tree of life is appliead to every single tick instead of over the duration of the HoT. This means that in it's current form I'll give all party members in range a buff that will heal them for 1035.

Applying the ToL effect fully on every single tick seems a bit overpowered to me. Does anyone know if this will be changed in the patch or if the functionality remains the same.

Just tested it, in casterform with Amplify Magic (Rank 4), no effect at all. They still just healed for 150 a tick.

Further tests you might try (if you want):

1) See if Fel Armor increases the heal amount for the warlock (HEH)
2) See if someone else in the party using the drums gives the same bonus when you go to tree form.

Why would you care about the healing effect? The mana one is the most important part of it. Is the mana gain affected by any +healing/damage/whatever?

I believe its effected by your +healing from gear. I don't know if ToL adds even more but I was getting ticks of around 169 with max ticks of 171 in my elemental gear. I did not have a tree druid in my group.

Below are results from 3 different wow web stats parses.

Heals Total % Ticks Avg Max
Drums of Restoration 2,038 10 % 12 169 171
Drums of Restoration 2,301 1 % 13 177 177
Drums of Restoration 2,912 1 % 13 224 224

Yes, the ticks of 200+ I experienced were most likely due to having a warlock with Fel Armor in my group. All incoming heals to that warlock get that bonus.