With the new holy tree, it isn't so bad, or so I've heard.
This is your basic full shadow path:
http://www.worldofwarcraft.com/info/...02323100511251
Your ideal combat goes something like this:
Mind Blast (pulls)
Mind Flay (damages and keeps him off you for a bit)
Shadow Word: Pain
Psychic Scream (he's on you by now)
Vampiric Embrace (to recover lost HP and take up time until you can cast...)
Mind Blast
Mind Flay
Power Word: Shield (fill some time preventing future pain)
Mind Blast
Mind Flay (he'll be on return sometime during this)
Mind Flay
Mind Blast
Mind Flay
Mind Flay
Repeat from psycic scream, replacing VE with SW:P
Of course, for this to work right, you need plenty of room to fear. In closer quarters, combat goes like this:
Mind Blast
Mind Flay
Vampiric Embrace
Shadow Word: Pain
Power Word: Shield
Mind Blast
Mind Flay
Mind Flay
Repeat from mind blast, recasting PW:S and SW:P as necessary.
If you're just putting into shadow to make leveling not suck, you only need Spirit Tap, Improved SW:P, Mind Flay, and the 3 points in Improved Mind Blast (6.5 seconds is just long enough for a real human to do two mind flays).
Since you'll be expected to heal in groups, I suggest putting lots of points into discipline, which helps you both solo and in groups.
This is my personal end-game raiding build:
http://www.worldofwarcraft.com/info/...32004100501200
While this is not a damage build, it does allow me to do fair damage when soloing. The role of this build in a raid is to heal a DPS group (usually rogues with a hunter with trueshot aura, and if you're unlucky, a warrior with improved battle shout--believe it or not the warrior takes the most damage...) and to keep up shadow weaving for the warlocks (they'll love you for it). Above all, you keep your group alive, hopefully mostly by VE, with the occational flash heal or prayer of healing; when your party is good on HP, you only mind flay since it is more mana efficient and less aggro than Mind Blast. On the issue of aggro, you'll have lots of it between the damage, the healling, and the massive over-healing, so don't think you can get away with cutting the threat reduction talents or take a blessing of wisdom instead of a blessing of salvation. With 5 points in Mental Agility and a group that usually doesn't take tons of damage, you are also designated decurser for fights that need it. If you decide to go this route, note that you should never introduce yourself to a prospective guild as a "Shadow Priest" (which carries a strong connotation of useless); with this build, you are a "VE healer hybrid".