There is an increasing percent bonus to damage done by Corruption and a +1% bonus to Seed of Corruption's chance to critically strike per point in this talent. See the table above for a complete breakdown. This talent is passive and is always on, provided that you have it trained; it also consumes no mana, nor does it cause your Corruption spells to cost any more mana.
Prior to the recent 3.0 patch, this talent reduced the casting time of Corruption by 0.4s per rank, with the final rank making the spell an instant cast. As of 3.0, however, Corruption is instant cast by default, completely disregarding any talents.
Speculation and Advice
In its new form, this talent is no longer a 'must have' for (almost) all Warlock builds. However, being that Corruption is one of the Warlock's bread and butter abilities, the talent still has a place in many builds. It may be prudent to weigh the value of the spell hit bonus and mana reduction offered by Suppression, especially for non-Affliction Warlocks who want to climb up a few tiers into some of the higher talents in the Affliction tree.
Ultimately, only time and testing will be able to conclude what the best course of action is when it comes to this talent at level 80.
Patch 3.0.2(10/14/08): This talent was completely revamped for the release of Wrath of the Lich King. It no longer reduces the casting time of the Corruption spell, which is now instant cast by default. It has been changed to a 2-20% dmg bonus on Corruption along with a 1-5% crit bonus for Seed of Corruption.