Horsemeat is indistinguishable from beef in taste. It is much leaner than beef; very close to buffalo. However, changing the diet may developed a more marbled cut of horse if its digestive system could handle it.
I found it sweeter than beef. Ate it in a truck stop cafe many years ago. My brother and I (kids at the time) asked our parents what it was and both said it was horsemeat, that they had often eaten it as children. I, of course, immediately started bawling, but even then had to admit that it was actually quite good.