David Frum nicely sums up my concerns about Justice-designate Miers. I encourage you to read the whole thing, I'll just excerpt a bit:
So the question must be asked, as Admiral Rickover once demanded of Jimmy Carter: Why not the best?
I worked with Harriet Miers. She's a lovely person: intelligent, honest, capable, loyal, discreet, dedicated ... I could pile on the praise all morning. But there is no reason at all to believe either that she is a legal conservative or - and more importantly - that she has the spine and steel necessary to resist the pressures that constantly bend the American legal system toward the left.
I am not saying that she is not a legal conservative. I am not saying that she is not steely. I am saying only that there is no good reason to believe either of these things. Not even her closest associates on the job have no good reason to believe either of these things. In other words, we are being asked by this president to take this appointment purely on trust, without any independent reason to support it. And that is not a request conservatives can safely grant.
There have just been too many instances of seeming conservatives being sent to the high court, only to succumb to the prevailing vapors up there: O'Connor, Kennedy, Souter. Given that record, it is simply reckless for any conservative president, especially one backed by a 55-seat Senate majority, to take a hazard on anything other than a known quantity.
But here is what we do know: the pressures on a Supreme Court justice to shift leftward are intense. There is the negative pressure of the vicious, hostile press that legal conservatives must endure. And there are the sweet little inducements - the flattery, the invitations to conferences in Austria and Italy, the lectureships at Yale and Harvard - that come to judges who soften and crumble. Harriet Miers is a taut, nervous, anxious personality. It is impossible to me to imagine that she can endure the anger and abuse - or resist the blandishments - that transformed, say, Anthony Kennedy into the judge he is today.