Review about Galapagos Islands
Go in May or June. There are plenty of reasons and explanations as to when you should go but here's my thinking - the water's fairly warm, the weather is good, the seas aren't too choppy, the animals are great (including the mating boobies) and it's all in low season which will save you a little bit.
If you are interested in a general visit to the islands i.e. you are not an expert in marine life or birds, but just want to see everything you can imagine, then May/June is great. I was there mid-May and was lucky enough to watch albatross nesting and blue footed boobies mating. On top of this, and I'm not sure how much of this is seasonal but I spent a long time snorkelling with turtles, sea lions, sharks and even penguins. We saw sea lions, fur seals, land iguanas, marine iguanas, rays, sharks, dolphins, giant tortoises, etc, etc...When I say saw, what I mean is that they were there, within inches. Not being perceived as a threat to the animals really can change the way you look at things.