I have to say that I agree with the fact that Discipline is definitely not that underpowered as most claim it to be. It could use a small buff in regards to raw output, but we're getting that in 4.0.6 anyway. I've always played disc, and I absolutely love the diversity of the spec combined with the very powerful cooldowns like barrier and pain suppression. I only have a holy offspec for those fights where it simply outshines discipline because of fight mechanics (for example al'akir, atramedes, chimaeron - mainly for body and soul, and chimaeron because holy is superior in aoe output). But every other fight I have succesfully duo or trio healed as discipline.
What many do not realize is that Discipline is all about managing your cooldowns and adjusting your raid accordingly as well. A well placed/timed power:word barrier can mitigate a TON of damage to the raid, yet the meters will never show this. But any good raider will understand how powerful the disc spec is.
In short, if you feel like you under perform in regards to output, try to remember that discipline brings more to the table but raw output. It feels slightly underpowered now compared to wotlk-bubblespam days, but that's because our bubble absorbs roughly the same amount as in wotlk, with quadrupled healthpools making it next to impossible. But this is getting 'fixed' hopefully next week.