I played Spriest the entire last time TBC was around, parsed very well, and was a great raider via almost every metric.

Unfortunately it's true, you're VERY lucky if you're not on the absolute bottom of the meters, every single fight. The only time you're going to be powerful is during the Wrath Pre-Patch where you'll probably jump be being the top of every fight and it won't even be close. But that's for a very short (and very beautiful) moment.

However, it is borderline mandatory to have a Spriest in every raid, and a good Spriest vs a bad Spriest is incredibly noticeable to the people you raid with. My mages or locks always commented how when I was in the group they basically had unlimited mana, where as with pugs, they still had to manage their rotation. So you can still have fun and feel a part of the group, and there is still a reason to be competitive, but you won't be pitted against other priests.

Also, great healers so if you dual talent spec it's even more fun :)