Short answer: A great place to start if you don't know about Latin America, but also a piece of activist journalism that has a very clear bias.

Longer answer: Eduardo Galeano was not a historian. He was an activist journalist. He had very clear sympathies and objectives when it came to politics and writing was his medium to change views. A lot of times he would oversimplify things or omit factors altogether because they weren't convenient to the specific explanation he was trying to give. He also uses anecdotes as sources, which can lead to repeating as a real event stories that are largely apocryphal.

With that said, I would recommend reading Venas Abiertas and his far superior trilogy Memoria de Fuego, which is the same thing, but better written and with a more polished and evolved version of the format he used in Venas Abiertas.

A good thing to keep in mind when taking into account the flaws in Venas Abiertas is when it was written (late 1960's, early 1970's). Dictatorships abounded in Latin America, many were being killed or imprisoned for their beliefs, many of the worst abusers had the US government's seal of approval, or at least a lack of American pressure against abuses. Venas Abiertas isn't supposed to be an academic, sterile reflection on Latin American history. It is a tool, a wake up call to young Latin Americans about how the region has been exploited by foreign powers, starting with Spain and continuing in the present with the USA.

You lose nothing by reading it and, honestly, the fact that it has been so influential to so many Latin Americans means that everyone should read it at some point.