As a level 40 priest, I can tell you a few advantages of being a level 20+ priest.
1) you'll never want for a party. everyone needs a healer.
2) with the right spec, you can either be one of the greatest healers in game, or the greatest damage dealer as far as casters go.
3) you have spells to protect you from taking physical damage.
The disadvantages of being a priest.
1) very hard to solo. (to get around this, make friends with a warrior or paladin, and quest in a party of 2 to reduce XP drain)
2)lack of variety in damage dealing spells.
3) can only wear cloth armour.

Personally I find the challenge of being a priest to be very interesting, and part of the fun in my game is to see if i can really take out that elite on my own.... and quite often, i do.
If you choose to create a priest who's sole purpose is healing/ buff party members... i strongly suggest you try a holy/ discipline spec. Personally, I prefer shadow spec, just because shadow form looks damn awesome, and all my armour is centered around maximizing damage i deal via shadow. it's just beautiful to crit for 400 as a priest. warm and fuzzy feelings inside and all.

This is probably one of my least used spells. Think about it, for a lot of mana you get to burn someone else's mana (only burns like 15% of what you spent to use it)? No thanks... I'll stick to Mana Burn if thats what I wanna do, and if I wanna do damage then I can melee attack for more... jesus, this is only a good spell for desperate situations where you have an abundance of mana...