Honestly he's one of the forgettables, overshadowed by his political mentor Andrew Jackson. The most interesting thing about him is that he's the only president whose first language wasn't english (dutch). Kind of the Carter of his day, his post presidency was probably better than his presidency. Economically his term was problematic, and he's criticized for the panic of 1837. He was pretty hands off with slavery in an effort to forestall conflict, refusing Texas entry into the Union as a slave state so as not to create division. However, after he left office he became increasingly outspoken abolitionist and supported Lincoln nearly 3 decades after his presidency.

He continued the Indian removal policies of Jackson, which caused the Seminole Wars - and most people would want to give him an F straight off. However, the Indian removals of the Jacksonian democrats weren't motivated by land grabs or racism per say. Indian removal was actually an attempt to save Native American culture, and while deplorable it was seen as the most realistic option. The Jacksonian and by extension Van Buren view would have been something along the lines of "White conquest of Native American lands in the east is unstoppable by the [much weaker than today] federal government. Native Americans are technologically backwards savages who have no legitimate hope of defending themselves from western expanding settlers. If they stay in the east, there will be a genocide or their culture will be assimilated by white culture, so we need to save them by moving them west of the Mississippi."

So presidency, he's probably a C-. Post presidency, probably a B+.