It depends on who and what you read. Since they became controversial, I notice them more. Charles Dickens used them both regularly--most pages seem to have both.

Virginia Woolf's writing has the most semi-colons I've seen and almost as many em-dashes. It fits her stream of consciousness style where there are very few hard stops.

Jack Vance used semi-colons in almost the opposite fashion to increase the tempo by having short clauses without using conjunctions. His action scenes are sometimes almost staccato.

Just today I'm reading Patricia McKillip and noticed she also used a lot of em-dashes.