we usually have 2 shadowpriests when we do kara and they go in the same group as the 3 healers. this gives us healers massive amounts of mana regen and on most boss fights we come out with 90% mana left. our shadowpriests are always in the top 3 places on the dps meters, they could do even more damage but their spells aggro like crazy so they can never go full out.
i've recently respec'd my holy priest to shadow and they're a lot of fun to play at high level. i tried heroic mech with it last night and i did insane amounts of damage but couldn't control my threat (think i need practise!) so i died a couple times. by using a shadowpriest we lost a potential crowd control because mind control isn't particularly trusting but we had a strong dps so the mobs went down fast enough for the tank not to take too much damage. the health and mana regen my group got from me really helped out on the bosses and when our healer died on the last boss i took up the healing spot.
not tried kara as a shadowpriest yet but will hopefully get a place next week. i recommend leveling your priest, but you will likely be expected to respec holy when you get to 70 =[ don't let your guild bully you into it! shadow's great =P