I've discussed this quite a bit on the official forum, and from ingame experience I am very pleased with my results of using flash heal instead of renew.
May I first say, this takes into account the following:
You are using the Flash Heal glyph, for -10% mana cost.
You have geared yourself for a reasonable amount of spell crit (including 5/5 holy specialisation)
You have the following talents: Serendipity, Holy Concentration, Surge of Light, Inspiration, Holy Specialisation, Test of Faith.
With this I am finding flash heal to be a good mana efficient heal, which makes good use of the spell crit stat, giving you free greater heals, flash heals and a good mana return from serendipity (25% if you cast appropriately)
Renew might have its place still, but after spending a huge amount of talents on focusing around spell crit, using renew to heal seems silly when flash can proc so many good benefits.
This is my current build: http://eu.wowarmory.com/character-ta...herhood&n=Nina
Note the 5 remaining talents, 1 is going into spiritual healing for 5/5 and the rest 4/5 divine providence