Esp. since the Notre Dame still has the crippled unfinished main towers for ages, and looks much better with those. On the other hand, they decided to finish the Gaudí towers, and they really dont look that well. They should have left it at the Notre Dame half-stage

It's definitely high up on my list of things to see. If anything, I'm worried my expectations and excitement level are too high and can only be let down.

Trust me, you don't need to worry about that. :-)

Get tickets well in advance.

Not to push your expectations even higher, but visiting and seeing the Sagrada Familia literally moved me to tears. It's one of only two buildings that have ever done that to me. (And I'm not religious at all, and was not raised Christian.)