Bernstein counsels:

I'm pretty confident that this suspicion of ambition is what's at the core of the unease with Kagan expressed by David Brooks and Andrew Sullivan (see also an interesting reaction from Ta-Nehisi Coates).  My advice for them is simple: get over it. 

Of course SCOTUS candidates are going to be ambitious and organize their lives around the pursuit of a seat on the court, just as presidential candidates, or Speakers of the House, are going to do the same.   It's not just an effect of the post-Bork rules, either; change the process, either by reforming the rules or otherwise, and you'll get highly ambitious nominees who play by those rules.  Make strong public position-taking important, and then you'll get people like Kagan and Roberts who publish lots of provocative arguments.  Same ambition, just tailored for whatever the norms demand.  Of course, presidents used to put actual politicians, people who had run for office, onto the Court, but I think we can safely assume that Taft and Warren were just as insanely ambitious as Kagan and Roberts.

It's not ambition that concerns me in any specific case. It's how people handle their ambition, and whether they act ethically within its demands.