I want to agree with most of the sentiment here, that Serendipity is pretty much a must have talent.
Like Worshaka said, it CAN promote poor play, and the old Serendipity is an even better example of how a talent can promote poor play; however, that doesn't make it a bad talent, it just means that a player needs to resist the temptation to play poorly. You can still use proper spell selection and still usually end up with at least a 2-stack and when you need to cast a PoH, and most usually a full stack
Also, I actually think Serendipity is a talent that is a reason for Priests to put Greater Heal BACK on their bars. With Divine Fury and a full stack of Serendipity, Greater Heal casts in just slightly slower than Flash Heal (1.6s with no Haste). At times where you are fairly sure you won't need to use PoH, or when you need to cover a dead/DCed Healer (and I think, generally, Holy Priests are probably the best equipped to briefly cover someone else's responsibility), it can be a life saver. For instance, we were doing Ignis in 10-man last night and the other healer DCed. It may have burned my mana a little bit, but rather than saving my Serendipity stack for the Flame Jets, I found that I was often using it for Greater Heal to help with the spike damage on the tank or when I needed to catch up from spending a GCD or two on the raid or Slag Pot, and I had little trouble keeping up with the AOE damage just fine with a with CoH and PoM mitigating the threat to the people that needed healing most while the slower PoH followed.
Anyway, my general philosophy is that what makes a Priest shine is that we have such a large toolbox, and anything that can be done to improve that is generally a good thing. This is a talent that does just that.