The rush seats for Jersey Boys at the Curran are the front row, starting at the left of the center block, working to the right. Sometimes they do sell the far corner right seats, which are right under the speakers and afford an excellent view of the spiral staircase, and the box seats, which I haven't been in for this show.
Is the sightline obstructed? Yes. You can't see many things which occur at the back of the stage. However, nothing of drastic importance (unless you consider the monkey dance, Crewe in the soundbooth, bowling or the end of "Dawn" critical to your enjoyment) is missed. It's $25, and you're practically in the show. I feel that the energy you get from sitting that close to the actors more than makes up for not being able to see Crewe's yellow couch, or whatever. People who just have to see everything may not agree.
'"Contrairiwise," continued Tweedledee, "if it was so, it might be; and if it were so, it would be; but as it isn't, it ain't. That's logic."'
~Lewis Carroll