I re-speced about level 64. I had run Hell Fire Penn, Zanger, and Terokkar, and part of BE Mountains, all the while picking the healing gear items from the quest rewards (the goblin town in BE MnTs has several good pieces). I wanted to learn a bit with the new build prior to lvl 70.
Did is slow me down - yes a little.
Was it viable - yes, definitely. Learn which seals and judgements work on certain monsters. Learn when to go for 100% chance crit, when to stun, and when to DPS vs. raw (righteous) damage. I used a more melee-centric weapon with the +crit, +damage, +healing gear. I kept a healing weapon in my bag for when I was healing only.
Was it as fun - not at first as I was kicking *** pretty quick with Ret build, but as I learned how to play the new build it is just as fun. You can stand and take a beating from several mobs your own level while healing and burning them down (concecration). I was also able to kill longer (grind) without stopping to drink.
Couple of things - go back to a area of lesser level monsters and run into the middle of them. Start burning and healing. It was counter intuitive to stand there and wait out the damage, but you do seem invincible. I went to plauge lands as the undead are plentiful, dont hit so hard, and Pally's mow them. This was more a mental thing...I think, but getting used to getting hit while 'playing your game' was not something I just picked up immediately. Bless, judge, seal, burn'em, heal, etc. Get your button presses or processes burned into your mind. Later, when the **** is hitting the fan, you will be much cooler and aware and able to handle the stress.
- start healing instances asap. There is so much more to pay attention to when healing, removing debufs, applying blessings, etc. You need the practice soon and a lot of it. You can also go to the Ring of Blood (I think that is what it is called - by the ogres/north central area) in Nagrand for the instance boss fight-like feel if you cannot get into instances.
- level up your first aid.
- look at some of the healing/buff addons. Grid and pally power come to mind. These will help you manage the large amount of data you will be processing.
- get your key bindings straight and practice (as mentioned above). The more automatic and practices they are, the better job you will do.
- have fun and make friends. A good healer is invaluable and people remember you.