Frederick Douglass felt that John Brown's raid on Harper's Ferry, Virginia was admirable, but dumb. He thought that it could not help the cause of the abolition of slavery and that, therefore, it was not a good idea. Douglass thought that the raid would make the whole nation angry at Brown and at the abolition movement because it would represent abolitionists waging war against the US government.
Although Douglass thought it was a bad idea, he admired Brown for doing it. He talks in one essay about how Brown was a hero for laying down his own life to fix an evil. He compares Brown to historical heroes such as William Wallace (the guy from Braveheart) and William Tell -- people who risked or lost their lives in the service of right and justice.
So Douglass thought that the raid was a bad idea on the tactical level, but he really admired Brown's courage and convictions.
Frederick Douglass was in awe. He had dinner with John Brown on a couple of occasions, and was utterly impressed with the fact a white man would go to such lengths to fight black slavery. When he was sentenced to death for treason against Virginia, and sent to the gallows to be hung, he still used his trial and hanging as opportunities to speak out against slavery.
In his newspaper, The North Star, Douglass wrote of John Brown after his death:
His zeal in the cause of freedom was infinitely superior to mine. Mine is as the candlelight, his was as the burning sun. I could live for the slaves. John Brown could die for them.
It's a good example of how polarizing a figure John Brown was. Southerners despised him, and thought him an example of what all northerners believed and were like. (A southern newspaper in Georgia had an editorial which stated: "An undivided South says let him hang") Northern abolitionists saw him as a heroic figure who had fired the first shot in the final battle against slavery.