Ok, it took many years for me to actually get round to booking my trip to Galapagos, and eventually I decided to do the deed this year. I did my research, and decided that March / April seemed to be the best time to go - low winds, calm water, warm water, clear weather, etc. as it's not a cheap trip, and most of the cost is in the travelling, I wanted to see all of the islands, and experience the diversity, but there were no 2 week trips available at the time I wanted to go., so I contacted Exodus and asked if they could put 2 separate trips together, and they arranged this without fuss or unnecessary expense. The service was exceptional. It turns out that the dates listed for the "wildlife cruise" are just weeks where the boat has been fully commissioned by exodus - you can go for 2 weeks anytime, and will still see all the wildlife, but there will be other operators' clients on the boat as well. For my trip, that wasn't a problem, and we had a great mix of people from UK, Canada, Italy and Israel.
The trip was led by a local company, and depending when you go, you may get different leaders; both of my weeks were led by Darwin who, as others have said, is amazingly knowledgeable on all things Galapagos, wildlife and geology.
Accommodation on the Cachalote is great, if a little cramped in the cabins, but the food is second to none, and the chef would put many hotel chefs to shame with what he manages to produce in a small galley.
I'm not specifically a wildlife enthusiast; I generally treat the wildlife as part of the scenery, and that's exactly what you get on Galapagos - sealions, marine iguanas, boobies (with all their foot colours!) and weird fish are everywhere, along with penguins, rays, turtles, tortoises and all sorts of other creatures.
Each day is both active and relaxing, in a strange kind of way. A topical day starts after breakfast with a 2 hour panga ride to the beach, where you may only walk a few hundred yards, but you get to take in an enormous wealth of wildlife and information. Then it was back to the boat, collect snorkelling kit and go for a swim with turtles, penguins, sealions, rays, sharks and weird fish; after about 1.5 hours in the water, it was back to the boat for lunch and onward journey to our next port of call. Then, later in the afternoon, we would have another 1.5 hours in the water, followed by another trip on land. Some people, especially on the second week, couldn't swim and while that's not necessarily a problem, it will mean you miss out on half the activities, as you can't go very far without the guide, and as most of the snorkelling trips were in open water from the panga, it's not always possible to sit on the beach. I wouldn't say I'm a strong swimmer, but I was easily able to snorkel for several hundred meters, so give it a go - you'll miss so much if you don't.
On the subject of snorkelling, if, like me, you wear glasses, consider investing in a prescription mask and a good snorkel; although you can use the boat's masks, snorkel and fins, I would always want my own snorkel, and the prescription mask was amazing. In spring, you won't need a wetsuit, but can hire one on the boat if you want.
I've written an article about my trip, which you can find at http://www.ksheard.plus.com/travel/galapagos/galapagos.html (assuming the link isn't removed from my post)