According to the last poster, they might have changed things, but we had 2 bike cases and 2 suitcases and there was no additional charge for our stuff.
Our boat wasn't crowded, nor was it dirty. We sat inside on seats because the ocean air was a bit chilly.
Our trip was 2 years ago, so perhaps things have changed, but we didn't have any of the issues mentioned above, it was an enjoyable way to get to Sorrento from Naples, less crowded for us than the train.
Keep in mind, we were coming from an overnight ferry out of Palermo, so we were right at the docks. We did have to walk a bit to get to the "commuter" ferry from the larger boat we just left, but it wasn't a bad experience.