INNER JOIN vs WHERE

Hello,

I've just had a quick search through the archives but couldn't find
what I wanted, so I've joined this list to continue my search for an
answer.

Is there any rule of thumb about how much more efficient an INNER JOIN
is compare to a regular WHERE statement?

I have a couple of queries in PostgreSQL that use a variety of tables
(four or five) linked together by key/foreign key conditions all ANDed
together. My co-worker re-wrote one of them using the INNER JOIN
approach and I wanted to find out if this would empirically improve the
performance.

I have not tried to do an EXPLAIN ANALYZE yet but I will try that.
Thanks for your responses.

Alex