Hey, I'm the healing officer of end game (heroic) 25 man raiding guild. I'm a resto druid myself, but I will give you my views.
Any fight where we are not to spread to much our Shammies does really good. Thou healing meters (wol) is not everything, it does tell a story. On our logs its usually me (resto druid) at the top with a shammy followed closely by Paladins, Priests, other shammies & other resto druids, but it does swop around depending on the fight. Some fights fav holy paladins, disc priests etc...
To me the value of a shammy is immense, aoe healing is great, totem is brilliant and mana tide is a must, specially new intense fights.
We used to have 2 really good shammies and all difficult fights I took to them if setup allows because, 1) they are really skilled 2) 2 x mana tides can have us really push the healing big time.
I can tell you from recruiting experience that any spec of Shammy seems to be few and far between (specially good ones) these days.
Its been the hardest class for us to keep at good numbers since the start of Cata
Resto shammy is also pretty easy spec to play and get really good output. I've head ppl complaining in the past that they are lacking behind other healers, but I guess that was before the 1st patch as I only see them doing really good in our raids.
On the other hand I have also had Resto shammies quit on me and rerolling as they did not enjoy the spec so much anymore (these are all old healers). Dunno what they would mean to you.
If I can get 2 more good Resto Shammies to add to my roster right now I would invite them asap
