The tests in my largest code work at a very high level. They are a number of example documents, chosen to exercise particular features, which are processed by the program and compared with known output. The tests have been pretty effective at spotting changes in behaviour and crashes, so I've found them useful. I'm not convinced that any lower level object or function verification would be particularly helpful, but of course programs vary in their structure. Treating the program as a black box with a variety of functions is a helpful way to test I believe.
Agreed. I've found system testing, using things like HtmlUnit, of things the user sees to be a much better use of time that testing individual method / function / subroutine calls.
For some types of applications, you might have to make your own testing tools, though: e.g. - compiler/tokenizer might need a tool to dump & examine the generated binary; batch job file-watcher & scheduler might need a tool to supply input files and manipulate apparent system time; ...
