So, not too long ago I wrote up a quick little comparison as to what exactly an Enhancement Shaman adds to their group over the non enhancement variety. I know a lot of people are confused as to whether or not they are worth bringing over a rogue or dps warrior, and hope this helps make things a bit easier to figure out.

Strength of Earth: +86 STR, talented +98 STR

Untalented (Resto/Elemental spec)
- Rogues: 86 Attack Power
- Warriors/Shamans/Bear Druids/Paladins: 172 AP
- Cat Druids: 206.4 AP

Talented + Unleashed Rage (Enhancement spec)
- Rogues: 107.8 AP
- Warriors/Shamans/Bear Druids/Paladins: 215.6 AP
- Cat Druids: 258.72 AP

Grace of Air: +77 AGI, talented +88 AGI

Untalented (Resto/Elemental spec)
- Rogues/Cat Druids: 77 AP
-- Rogues: 1.93% Crit, 3.85% Dodge
-- Warriors: 2.33% Crit, 2.57% Dodge
-- Druids: 3.08% Crit, 5.25% Dodge
-- Shamans/Paladins: 3.08% Crit, 3.08% Dodge

Talented + Unleashed Rage (Enhancement spec)
- Rogues/Cat Druids: 96.8 AP
-- Rogues: 2.2% Crit, 4.40% Dodge
-- Warriors: 2.67% Crit, 2.93% Dodge
-- Druids: 3.52% Crit, 5.99% Dodge
-- Shamans/Paladins: 3.52% Crit, 3.52% Dodge

Totals
Unleashed Rage is +10% AP, so for every 1000 AP you have, including other buffs and potions, you gain 100 AP. Lets assume that every melee class has 2000 AP buffed, so Unleashed Rage would add another 200.

Untalented Totals (Resto/Elemental spec)
- Rogues: 163 AP
- Cat Druids: 283.4 AP
- Warriors/Shamans/Other Druids/Paladins: 172 AP
-- Rogues: 1.93% Crit, 3.85% Dodge
-- Warriors: 2.33% Crit, 2.57% Dodge
-- Druids: 3.08% Crit, 5.25% Dodge
-- Shamans/Paladins: 3.08% Crit, 3.08% Dodge

Talented Totals (Enhancement spec)
- Rogues: 204.6 + 200 = 404.6 AP
- Cat Druids: 355.52 + 200 = 555.52 AP
- Warriors/Shamans/Other Druids/Paladins: 215.6 + 200 = 415.6 AP
-- Rogues: 2.2% Crit, 4.40% Dodge
-- Warriors: 2.67% Crit, 2.93% Dodge
-- Druids: 3.52% Crit, 5.99% Dodge
-- Shamans/Paladins: 3.52% Crit, 3.52% Dodge

The Shaman Tier 4 2 piece bonus is +12 Strength on your Strength of Earth, which together with the improved Strength of Earth Totem, brings you to +112 Strength. Or an additional 14-28 (15-30 with UR) attack power for your whole group.

The attack power bonus will scale perfectly with gear. As a basic rule of thumb, enhancement shamans provide around 250 AP, 0.5% crit, and 0.5% dodge over resto/elemental shamans. Keep in mind that a flask of relentless assault is 120 AP. You can't stack pots in 2.1, so that attack power is proportionally more valuable.

These calculations are all before bloodlust or especially windfury which scales dramatically better than agility with warriors, paladins, and combat rogues.

Original post expanded on a bit by Slant in his excellent Shaman F.A.Q., which you should all read for this and much more information about the class