I believe that the use of animals is often preferable to the use of humans. Take for instance the study of genes and genetic mutation. Nobody cares about the use of fruit flies for that work because fruit flies aren't cute. Use a mouse or a rabbit of a lamb and people start freaking out. Why? It's partly been bred into us over millions of years, we react in certain ways towards mammals in pain because we recognize specific signals. Some of these signals are obvious, while others are imperceptible. Secondly, are reactions are fueled by our culture and social mores. We react a specific way because we believe that we're supposed to react a certain way. In the pursuit of science we attempt to disregard emotion and emotional responses as well as challenge the social constructs that determine our behavior. Can you imagine what would have happened if Galileo had chosen not the question the order of the solar system, Steno the age and rate of soil deposition/erosion, or Darwin the origin of species. We'd still be in the dark ages. We cannot let concern for how we are being perceived influence the quest for the truth.
That having been said every animal life sacrificed in the name of science should be treated with the utmost respect and should only be taken out of necessity.