Since hitting 90 I've had some good and bad experiences as Holy, but they seem to be issues that I've been having on all my healers (I have all the healers at 85, with the exception of my Paladin which is at 90, and I haven't leveled a Monk on live yet) and that fellow guildies are having.
The issues primarily have to do with people thinking that this is Cata, and we can just bum-rush mobs and bosses without paying much attention to what's going on around them. When this does happen, I can keep people alive but it isn't the easiest (low spell-power and spirit consequences).
I'm having to really think about how I'm going to go about healing an encounter, which I like. Previously when my raid-leader would assign groups to heal, all of us healers would just be like, "We'll heal people that are dying." and it would be fine. Now, however, I'm having to be strategic with my heals so that I have big, powerful heals available when I need them.
Making sure that if I'm going to WoG, that it is 100% with 3 HoPo and no less unless it's an absolute emergency (and pray to the Blizzard-gods that the itty-bitty baby WoG crits).
Having 100% uptime of Sacred Shield on my tank, and NEVER letting it fall off (easier said than done).
Utilizing Beacon of Light when people are taking damage.
I'm noticing that I'm ending fights with anywhere between 30-40% mana (sometimes more, depending on the group), but my top heal is Beacon of Light, then it's Illuminated Healing, Holy Shock and then Holy Light.
Biggest downside I've seen (and something I'm seeing a few people mention) is that "greater" equivalent heals aren't really doing much when comparing it's effectiveness in Cataclysm. In order to top someone off that is practically dying with Divine Light, I need to use quite a few of them. HS + Divine Light just doesn't heal someone like it used to.
These are just my observations, and a lot of the issues will go away once people are used to their class and the encounters. I definitely wouldn't say that Holy is weak, because it feels very, very powerful if done correctly. I just don't think that I can say, "a monkey could heal with a Paladin" anymore, like I could say back in Cata. :P
PS: Warrior tanks are the most amazing tanks to heal. <3