For maintenance & extension, yes. For new development, no. I've found that unless I have a solid architecture already (such as in a mature product), I end up

For maintenance & extension, yes. For new development, no. I've found that unless I have a solid architecture already (such as in a mature product), I end up massively modifying, or even entirely rewriting most of my tests as development goes on, which is a waste of time. Or even worse, I end up avoiding modifications to the architecture because I dread the amount of test rewrites I'll have to do.