Assuming you accept the need and the role for a dedicated healer, the Healer class needs two things to be truly capable.

First, you have to spend a feat on Spontaneous Healer, so you can spontaneously cast the healing spells that are your bread and butter while still maintaining a useful toolkit of anti-poison and stat-healing spells. Were this a feature of the class, we would all be happier with it.

Second, the Healer needs a better spell selection. You need to acquire domains or bonus spells through prestige classes to be able to do something OTHER than healing people, and you need a generous DM to add the new healing spells in Spell Compendium, PHB2, and other sources to improve the class' repertoire.

In all, it makes a fantastic NPC healbot, but a weak PC.