As soon as you gain access to [Beast Shape II][1], which should be at 6th level for the vanilla druid, you turn into creatures that have the [Grab][2] ability.

> **Grab (Ex):**

> If a creature with this special attack hits with the indicated attack (usually a claw or bite attack), it deals normal damage and attempts to **start a grapple as a free action without provoking an attack of opportunity**. Unless otherwise noted, grab can only be used against targets of a size equal to or smaller than the creature with this ability. (...)

> Creatures with the grab special attack receive a **+4 bonus on combat maneuver checks made to start and maintain a grapple**.

So, unless you really feel you need the extra +2 from [Improved Grapple][3], you do not need to spend two feats simply to be an effective grappler.

[Greater Grapple][4], however, is an upgrade to your grapple checks, but again, your real benefit from the feat is the ability to **maintain a grapple as a move action** instead of a *standard action*. 

This will cost you three feats, from which you will only benefit partially from one and recieve the normal benefits of the other. Again, on a class that does not gain any bonus feats, so investing on **grapple feats are not very useful for druids**.


  [1]: http://www.d20pfsrd.com/magic/all-spells/b/beast-shape/
  [2]: http://www.d20pfsrd.com/bestiary/rules-for-monsters/universal-monster-rules/#TOC-Grab-Ex-
  [3]: http://www.d20pfsrd.com/feats/combat-feats/improved-grapple-combat-final/
  [4]: http://www.d20pfsrd.com/feats/combat-feats/greater-grapple-combat-final/