Many of the biographies I've read suffer from the same major problems. They're long, boring, and don't do their subjects any justice. Fortunately the Walter Isaacson biography of Einstein was none of these things, though it does suffer from the two traditional weaknesses of biography: First, where it repeats itself often, unnecessarily eliminating the possibility of suspense. Second, putting its subject on a pedestal.
Of course Einstein was not in any sense a terrible person, or a politician or leader, so being in awe of him isn't such a bad thing. However, I think Isaacson would have done better to show how Einstein mellowed as he aged. We all know that Einstein's ideas and politics changed, but it's unrealistic to assume a person's personality remained static throughout his or her life. While Isaacson readily and repeatedly noted Einstein's distance from people he failed to highlight that Einstein's rough edges could sometimes be quite sharp when he was younger. Take for example Einstein's biting remarks to Fritz Haber. Einstein was extremely disdainful of Haber's attempts to be a "good German", a fact Isaacson acknowledges. However, some of Haber's letters to Einstein lamenting his own status as a second-class citizen received rather cold replies. One can of course argue that this was deserved, but I think there is some difference in Einstein's approach to confrontation when he was older compared to when he was younger. Still, while too many biographers are quick to de-emphasize faults, Isaacson avoids this for the most part.
There is one aspect of the biography that disturbed me initially, but by the time I had finished I realized that I was working off of bad information. Isaacson talks a lot about Einstein's God. Perhaps it is a reflection on me more than Isaacson but I felt he spent far more turn on this than necessary. Though as I stated earlier, I had learned something I did not know. Based on a widely publicized letter and one that Richard Dawkins in particular flaunted--I got the distinct impression Einstein was a strongly avowed Atheist. However Isaacson makes it quite clear that this is not the appropriate conclusion based on Einstein's own letters and statements. Eisntein's own words make it quite clear that he was Deistic rather than Atheistic. Throughout his life Einstein only denied the existence of a personal God- however strongly he denied it.
I have never really felt Einstein's opinions on this matter to be particularly important regardless. Whether you're an active atheist or deeply religious. Einstein was wrong about any number of things (being quite the human being). Richard Dawkins made one good point in the recording I linked to above, that Einstein's views shouldn't be accepted on the basis of authority- whatever his views may have been. Simply acknowledging that Einstein was a genius without really comprehending the nuances of his specific brand of genius, just to associate him with a personal cause, is quite appalling.
As for a verdict on the book: In general it's a good biography that seems to prize accuracy and relies heavily on quoatations of primary source material to make its points for it. It's a little long, the price paid for being thorough- but the lulls and lags are few and far between.