Just finished a ZA + kara tonight, so my mind is still fresh about being a priest. I used to be holy and it was fun, healing in raids, smite spamming in heroics because the tanks were overgeared, taking on 3 mobs at once during dailies, etc. Then my guild died and I joined a guild looking for a shadow priest.
I can sum it up in 4 words (no particular order):
Melt. Faces. Laser. Awesome.
Currently I put out respectable dps. Sometimes that is enough to top the meter, sometimes not. But I don't care because it's not just about that. I'm consistently out-damaged by destrolocks currently, but with the other spriest on holiday at the moment I take satisfaction in knowing that 10% + 5% of their damage is thanks to me.
Also, apparently I'm a hot commodity now, everyone wants to be in my group. The mod "Recount" currently has a meter that records mana recovering abilities, and when I check it usually the only thing that gives people in my group more mana than my vampiric touch is life tap, and even that's not too far ahead.
But yeah, the damage scaling is bad. VT and SW:P have good coefficients (100% and 146% [talented]), and account for a lot of damage, but the direct damage spells are not nearly as good. Blast and death, aside from having cooldowns, have a small coefficient (42.86%), and even our iconic face melting laser, mind flay, is 57% over its duration. No other casting class has coefficients this low on their direct damage spells. This means that as gear increases across the guild and across the raid, your damage will of course increase, but will fall behind the other dps-ers.
As for BG, I did do a couple while gearing up after I respecced, so I could get the s1 staff (looks awesome in shadowform btw). As long as nobody saw me I was fine, I dotted up everything with impunity. Definitely squishier than when discipline spec though, but I did have more fun, though that could be because my experience with heavy disc was basically me heal-tanking while everyone else did the damage.
Gear seems alright, I mainly went for mage/warlock stuff, probably gimped myself by getting spelldamage over shadow damage. However, there's
this trinket that drops in heroic MgT, and it is godly (at least for me), shoots out baby shadowbolts when it procs.
But, if you are thinking about a shadowpriest, you should go for what is fun. Do you want to look scary and all shadowy and evil, and shoot blue lasers (yes I'm a laser freak :P) at stuff? Do you want to eat your enemies health and turn it into mana for your friends, so they can shoot more stuff at enemies? If so, then shadowpriest will be fun (and casters/healers will love you). But if you're after massive crits and being on top of damage meters in 25-man raids, then shadowpriest cannot deliver.
....also did I mention the lasers?