It has an on use that can be macro'd to your spells to make sure it is constantly active and providing regeneration. Overall, this is the best holy trinket in the game. According to WowHead's comments and math this trinket can be worth around 284mp5 factoring in an average number of SOW procs, mana tide totem, and regen from the crit this gives.
A trinket that provides high spellpower and a wealth of mp5 since it acts like an Illustration of the Dragon Soul, this is very powerful. 144 passive mp5. Better than Talisman of Resurgence if a fight exceeds 4 or 5 minutes.
From logs the on use is roughly 5-6% healing on an AOE damage fight like Sindragosa. If you replaced Meteorite Crystal it's 5% healing (this trinket) vs ~10,000 mana (2 Meteorite uses plus its intellect estimate, increase mana for longer fights like LK). This is a good trinket for fights where mana doesn't matter. Otherwise sticking to regen is my preference.
This is only one of the top 2 trinkets if you are min/maxing intellect.
This trinket is better than a heroic Solace if a fight is less than 4 or 5 minutes. After that, Solace pulls ahead in regen.
This short analysis ignores the fact that this trinket also provides a powerful 600 spellpower on use effect that helps during Divine Plea usage and other burst healing scenarios. When using Sacred Shield with this spellpower boost you can also lock in a higher spellpower coefficient for the duration of that Sacred Shield, absorbing more damage.
It provides over 60 mp5 base and the proc works with Divine Plea very well if timed. Benefits from replenishment increase with the added maximum mana pool. The proc also provides roughly 2% spellcrit and a good deal of spellpower. The trinket is still good because the proc carries it.
The 900 mana proc every 45 seconds equates to ~100mp5. The 2.07% crit turns out to be roughly 30mp5 assuming a 1.3 second holy light cast speed. This trinket still holds its own even with the diminished value of crit.
This trinket, after the 12/14/09 hotfix, now works with beacon heals and the Flash of Light hot. As for Glyph of Holy Light... it works. HOWEVER, it only procs twice maximum per holy light (not 5 like you'd expect). It's worth up to 150mp5 if you keep the hot up and beacon up. The on use isn't bad either. A decent alternative if you're willing to lose the intellect.
Really just a PVP trinket more than anything. It still has a good deal of regen (more than the 264 badge trinket). The heal is off GCD so it could be handy.
Originally thought to be a 1 minute cooldown, new evidence has surfaced that it was a datamine error and it is now 2 minutes. So much for a good ICC healing trinket.