I just took a minute to see what the best seats available through Ticketmaster are for the shows at Summerfest. As is always the case with Summerfest and the Marcus Amphitheater, the shows have sold extremely well.
The July 2 Roger Waters show is completely sold out and the only tickets remaining for the July 5 Bon Jovi show are general admission lawn seats.
For the other three shows, The Fray on June 30, John Mayer on July 3 and Toby Keith on July 8 reserved seating is still available, however not in locations of the Marcus Amphitheater I would want to spend my money on. If you don't already have tickets and you are looking for some, without working with a local ticket broker, you'll be sitting in a reserved bleacher seat for Toby Keith, in a (WAY) off-center seat in the yellow section for John Mayer and in an OK reserved seat in the yellow section. But with regard to John Mayer, this is where I would recommend doing your homework; if you spend just $30 more per ticket and buy tickets from a broker like Ticket King, you can buy Section 2 seats, just 30 rows from the cutie-patootie himself (and maybe Jessica too?). Sitting in Sec 2 is a considerable improvement and doesn't set you back as much as you might have thought.