All the super units from Starcraft 1 got buffed, but the Archon is left completely unchanged. Most players build Archons as an emergency fallback plan, only after the high templar runs out of energy. I think this is a sad state for the unit. It is not worth spending 300 gas on a subpar combat unit that gets 2-shotted by ghosts. The archon is a heroic unit, perhaps it should provide a leadership role to protoss units:

- Inspiring aura: The archon's presence inspires nearby protoss units to greater heights. This could include any number of things, such as increased speed, zealot health regen, or shield regen.

Meditate: The archon can meditate to regenerate its shielding far quicker than normal. During this time it is in communion with the psychic plane and behaves as if cloaked. Might help survive a ghost EMP.
Energize: The archon can sacrifice its shielding to regenerate the energy and shielding of other units. This can be extremely handy for escorting the vulnerable mothership, as protoss currently have no protection from focus fire. Think of it as being like a shield battery for late game armies, providing a large boost to hit and run strategies.

As we also said during the Twitter Chat with the Developers, we still consider the Archons as backup units and not as core units.
You should use them when your High Templars run out of energy, but you shouldnàt consider them as the bulk of your army composition.