How are shadow priests in WotLK?

Hi there!
After a few years of taking a break from WoW, i want to get back to playing and as always whenever i a new expansion comes or i change servers/realms, i have trouble choosing what to play. Currently, i'm debating whether i should play retri pala or shadow priest.

I've played paladin in the past and i pretty much know all the ups and downs for playing them. As for shadow priest, i don't know much and i could use some info on them before i make my decision.

Are they fun to play? Are they hard to play? Are there a lot of them on the Icecrown realm? How do they compare to other classes in PvE and PvP?

So you understand my way of thinking when picking a class, i will elaborate on reasons why i ask these questions. First i want to have fun but not have to be super focused on my dps rotation. Example of this i feral druid for me. I really enjoy playing them but their dps rotation is really punishing if you make a mistake. Why i ask for their approx number on the realm is so i know how much is the class needed. Example is paladin, who is from what i remember, one of the most played classes and therefore there is a lot of competition for raid spots and gear drops. Pretty much the question goes, how much is spriest needed in raids and are there a lot of them? And the last question about them in PvE and PvP is because i like doing both pvp and pve and i would like to pick a "balanced" class. When i say balanced, i mean at least that it isn't under performing much compared to other classes. It doesn't have to be the best dps, but that its decent.

What would you recommend for me to choose, for whatever the reason you believe i should pick one over the other. Whether you are an experienced priest player or someone who has never played priest at all, i would like to hear your thoughts about spriest and how you see them.
Thanks in advance :)

spriests are fun!

PVE:
you can be sure that a every raid needs a priest. if not healer then a dps. even 10 man raids want a priest in their group - the buffs are awesome, you can also os heal for some bosses if needed.
shadow priest provides nice dps, but dont expect to top dmg charts when it comes to spriests. the rotation is DOTs dependent (keeping them up) and channeling mind flays (and some mind blasts b4 4pt10 bonus). your dots do a lot of dmg, so it is less punishing to then a feral druid..
what i also love about spriests is being kinda independent when it comes to survivability in raids. having a spell (vampiric embrace) that heals you all the time based on your single target dmg done is awesome (and a smaller portion that heals the rest of your party).
i also find myself dpsing and helping rest of the raid quite a lot - when it comes to shielding a raid mate on dmg spike, or throwing in a heal when a healer dies.

in terms of items and gearing - priests are casters, so the competition is tough. you will have to rely on your /roll, or join a guild with DKP/EPGP system.

PVP:
spriests can pretty much beat all other classes. they provide solid competitive pvp experience. you have fear, disarm+stun, silence, mind control... you DOT up your enemy and see the HP goes to 0. (did i hear DP spam?)
you can outplay better geared players, especially since you have so much CCs in your arsenal. it requires some practice tho...

i think you should pick a class based on your joy to play it. warmane has been around for a long time, there are plenty of players in all of the classes. pick the class that you think you will have fun playing, not the class that is "needed".... what is needed the most is tank.. i am sure you will not have trouble finding raids as a tank.

if you have more questions feel free to ask.