I created a priest to play with my friend and eventually heal for his guild, but I'll be moving my main (a resto druid) to his server soon and I don't want to have two healers. I'm really enjoying my priest and may take her to 70, but I don't want to respec. Are shadow priests desirable for groups at 70? What role do we play in a group?

I've done some snooping and I think that a shadow priest would be great support for a group with a warlock in it, because we could increase the warlock's shadow damage AND regenerate mana AND health at the same time while still doing damage. I don't think it's the DPS that matters with a shadow priest, it's the bonuses that come with it.

I am a lvl 70 shadow priest.  and let me tell you, that if your gear is right we can DPS with the best of them.  but ur right a warlock might beable to out DPS us a bit, but i think we are more valuable due to vampiric embrace and vampiric touch.  and our misery ability helps all spell casters.  i mean  when ur ticking 495 and 475 with ur 2 dots every few seconds and critting them for 2800 with ur mind blast, and 500 every couple seconds with ur mind flay,  we can REALLY hit a boss hard.

That's exactly what I was thinking. I've heard a lot of healers on thottbot say they love having a shadow priest in the group because it basicly gives them endless mana.

Some endurance fights are made simple by having a spriest along.  Great dps, an extra cc, especially in Kara with the undead, and a mana/health battery.  What's not to love?

Shadow priests are about 160-200 mp5 when equiped w/ FSW and Spellstrike and kara epics to their party.  These is about as much/or more mp5 then healers have unbuffed while casting going into SSC/TK.

They also restore about 200 hps to everyone in their party.

Basically if you bring 2 shadow priests you can probably get away with having 1 less raid healer, since healers have more mana and they don't have to top off as much.

Shadow priests also give all caster dps 5% damage (misery) and locks get an additional 10% shadow damage debuff.

Oh and shadow priests put out lots of damage too, on par with most classes.  Get FSW, Spellstrike, Orb of the Souleater, Glad Weapon, PVP dreadweave, icon and just pwn.

yeh shadow priests are pretty good at DPS but other spell casters can DPS better with out taking aggro
i am a tank i rather take an warlock/mage rather than a shadow priest

All ppl say taht spriest are not good on dps... but that is not true! i /w some locks and rogues ( rogues and locks are what ppl normaly say that ar ethe bets dps so i asked them) and asked tehm if in Black tenple they made more dmg then a spriest tehy said me:

in Kara... and all till The Eye rogues and locks make the bets dmg... but in Black Temple spriets make more dmg .. a lot more... spriets and hunters..

i asked why?

they said .. Locks/Rogues are rly good till spriest and hunters get tier 6 when a spriest is with tier6 they cn make a lot better dmg then a lock with tier 6 awelll.. coz with bad gear locks/rogues win, with gr8 gears spriets and hunter ( and ofc rogues continue making good dmg but spriest ar ebetter and tehy heal/give mana bk to party while making awsome dps)

so stop saying that spriets suck! ffs ask some guilds taht make black temple and then u will see that spriets don´t suck tehy cn be the better class makign dps WHILE GIVING MAA /HP TO PARTTY!!! ;P =) ^^

I wouldn't say they out dps other classes in the same gear, even at t6.  Actually the t6 arrangement of stats favors more crit and hit, two things that scale poorly on a spriest compared to destro warlocks for example who have >85% of their attacks that can crit and do 200% damage when they do crit.  Their t6 set bonus also increases sbolt damage by 6%... which is basically a 6% dps increase.

But that isn't to say their damage is bad.  When equipped correctly:
focusing on roughly (post hit cap and 1k SD)  1 SD = 1.2 Haste = 6 crit

They do average dps (varies greatly based on player skill) but have huge raid utility.  In five mans they make sure your casters don't go oom, which is important because my holy pally hates to waste even 1 mana pot on a herioc.  I would bring one to every five man if I could for the buffs as well. 
Holy pally + spriest + lock + ele shaman + pally tank = ideal instance makeup imo, cc is for the weak.