It's a snap to get to Ostia from central Rome -- Metro B line to Piramide, and a quick and easy transfer to the Lido train line. It's all covered on a single Metro ticket (or Roma Pass), and takes about 30 - 45 minutes total each way. Then it's about a 5-minute walk from the Ostia Antica station to the site entrance.
Ostia is uncrowded and lacks the cheesy Pompeii-style souvenir stands. There is a nice cafeteria and museum on the site. Large stands of stone pine trees provide welcome shade and a pleasant aroma.
Ostia's demise and deterioration was gradual, while Pompeii was frozen in time by the volcano. Pompeii has more color and more decorations still in situ -- but to me, visiting Ostia, especially on a day trip from Rome, is overall a more pleasant experience.
Rick's 2012 "Rome - Back Street Riches" video (here) describes Ostia Antica in more detail.