NOTE: This reflects the current state of the game on 09/04/11.
Thanks to Kamakazees' report on Armor properties, I wanted to share my thoughts on Protection and Force Shield.
*** Protection adds +10/+20/+30/+40/+50/+60 armor to the whole party for 2 minutes. Rank 6 equates to ~1-2 damage points reduced per hit.
The armor amount does not scale with level, so at 20+, you barely notice the damage reduction. The only beneficial effect Protection has is a placebo effect on people who don't know how armor works. If you don't cast it, people may get angry with you or think you are an incompetent player.
*** Force Shield adds +20/+40/+60/+80/+100/+120 armor and +1 regen to your character for 30s, has a 45s cooldown, costs 10 Energy. The shield only works for the first 25/35/45/55/65/75 damage your character takes. Rank 6 equates to ~2-3 damage points reduced per hit. With Protection, that equates to ~3-4 damage points per hit.
Again, the armor amount does not scale with level, so at 20+, you barely notice the damage reduction. What's worse is that Force Shield only lasts for up to 65 damage. That's NOT damage blocked, but total damage. At level 20+ on Numa Prime, that can equate to 2-3 attacks. So best case, you only get ~6-9 damage points blocked for spending 6 skill points.
Force Shield's only redeeming quality is that it also provides +1 energy regen (or +2 energy regen at rank 4). This means you get 1 (or 2) extra energy point every 1 second tick. Since the skill costs 10 energy, it takes the skill 10s (or 5s) to recoup the energy cost. That leaves 20s (25s), or 20 (25) ticks of extra energy. End result: You spend 10 energy and get back 30 (60), for a net gain of 20 (50). This equates to an ideal effective energy recovery rate of 0.44e/s (1.11e/s)
That energy isn't guaranteed, though. If you get hit and lose your shield, you lose out on the potential energy. In these situations, you're better off just guzzling an energy potion.
It may make more sense to invest in Leech for personal protection. Leech says it lasts for 3x 2s ticks (but I've counted 4x 2s ticks). The advantage of Leech, though, is that it scales with your weapon damage. Even if only one rank 1 leech tick fires off, I was seeing +15 health recovered on my lv. 23 Engineer. That's more than the ~6-9 damage that a rank 6 force shield gave me.
If you are considering putting points into Force Shield for better energy returns, don't max the skill out! Consider either stopping at 1 (to get the base recovery) or stopping at 4 (for the 2 energy per second) and then investing 2 extra points in Revive. Revive provides a hidden 1 energy per second recovery rate buff for 60s. Bringing Revive to rank 3 reduces its cost to 10 energy and thus, increases its effective recovery from 0.67e/s to 0.83e/s.
Keep in mind, though, that these energy gains are short-term. The skill costs remain constant across ranks, but your recovery rate and your maximum energy pool will increase. According to Bronislav, you get an extra energy recovery point every 143 INT. That means with gear that provides INT, you're likely to see an extra point around levels 20-25 and 30-35 (assuming gear scales up linearly). You'll also be likely to find implants or armor pieces that add energy recovery directly. As the game expands, these gains might outpace the recovery benefits of Force Shield.
In summary, skills that provide a fixed bonus risk becoming obsolete with level. They do not track with the player. A better solution for these skills would be to make them increase by percentages. And for Force shield, eliminating the durability factor would make it more useful and reliable.
Regardless, in their current form, you get more bang-for-the-buck by putting points in other skills.