You should, but keep in mind that it's big and it'll take more than a couple of hours.
I remembered another museum you could visit sometime - The National Polytechinc Museum. It's on Opalchenska str. 66, near the Opalchenska metro station. It's rather small, but the exhibits are still fun and quite interesting - a lot of old clocks, some computers, food for astronauts... all Bulgarian, of course.
And I just have to correct myself - in the previous post I meant that we have THE oldest gold jewelry that's ever been dug up. :)