Maserman, basically to give you a straight answer Spriest dps is good. Its my main spec currently and i thoroughly enjoy playing one. The rotation of spells and priority of what to use when really makes it fun and you have to have some skill to play one well.. opposed to a warrior who has a whopping 3 button rotation or a paladin who can macro <next available spell /cast> to his mouse wheel and scroll back and forth and put out 4500dps doing so.
If you put forth some effort into getting yourself some good gear then youll go a long way. 2 piece 8.5 tier(tier 7 gear is ok.. but easily replaced), 15% more to DP (now rediculously easy to attain and a must) make sure your hit capped (290), get the bis belt and boots made for you (ran me about 1200g each) and go find a bis list somewhere (shadowpriest.com i know has one for 3.0) and work towards getting as many pieces as your hit cap will allow. Gem for spellpower ALWAYS, cept i use 21crit 3%crit dmg increase meta.
A Spriest's dps is largely based on his gear since we dont have out right burst like most classes and takes some time to get rolling. Do some research on a proper spec, which really isnt that hard to do on your own if you read, and find a good rotation.
Rotation is always up for debate at what works best. My rotation is this: VT>DP>MB>MF>MF clip last tick of MF for SW has added dps>MB>SW:P>MF>VT>MF>MF and so forth.. i put up SW:P late, but it gets the full benefit of Shadow Weaving and is actually our least powerful dot, so it priority can be put off a tad. Make sure VT and DP are ALWAYS on target since they are your strongest dots. Add VE on boss fights.
Its always situational in a boss fight of how much dps your actually going to do.. whether you have to do alot of running around or there are buffs given to you will depend on how low or high it may be. I average around 4500-5000dps in Ulduar25.. thats an average between all encounters. Some i hit only 4k and on fights like Hodir i sustain over 7k even w/ the running around.
So in conclusion.. is it worth it to go shadow? yes, but it does take some work on getting geared up enough to really make an impact.