Quote from danielbarronChromatic gave stats too. I will put it back but I still consider it a pathetic flask for most occasions. I have tried comparing damage taken with two holy paladins and it has made no noticeable difference - the one with it even took more damage.
Added in r231.
I understand where you're coming from with this, but actually given the new resist mechanics in WotLK, resistance flasks can be more useful than a health flask. Basically, there are fixed levels of resistance that one can have that will completely eliminate anything less then a 10%, 20%, 30%, etc. resist so using one can get you over one of these "caps". See http://elitistjerks.com/f15/t44675-resistance_mechanics_wotlk/ for more information.
In addition, the whole point is to catch people who are slacking, and I doubt that anyone would use a resistance flask for any reason other than actually needing it for an encounter.