Switch: A Comedy of Terrors by RivkaT. In which bodies are changed, explorations commence, and--oh God, I can't even begin to cover everything.
This is almost too good to be true.
Lex is just so Lex...God, is he Lex, in the wrong body entirely. Funny, fascinating insights and hijinks (I never thought I'd be using THAT word with a straight face), and everything that can go wrong and then again, doesn't.
Read it now. Now now now. On third run through, and damn, I LOVE this story.
Ooh yes. I'm blissing out here.