T = TIMEIT(F) measures the time (in seconds) required to run F, which is a function handle.
TIMEIT handles automatically the usual benchmarking procedures of "warming up" F, figuring out how many times to repeat F in a timing loop, etc. TIMEIT uses a median to form a reasonably robust time estimate.
UPDATED 16-May-2010: New option to specify number of output arguments to call F with. Overhead measurements now cached so most timings run faster.
UPDATED 31-Dec-2008: More accurate when timing very fast functions; warns you when the reported time might be affected by time-measurement overhead; calls F fewer times when F takes more than a few seconds to run.