It's certainly worth a second look - I only spent about three hours there and about half of that eating lunch at a sadly not seaside restaurant so I speak from admittedly limited experience.
I can't really say that for me the beach justified the trouble involved in reaching it, but if you're spending the night (or driving yourself) then I'm sure it would be a lovely mini-holiday.
And just for those who are curious - the trip involves a train ride from Termini to Fondi-Sperlonga - that part is easy enough - followed by a bus ride. That's the tricky part. It's supposed to be timed to the train arrivals but in fact comes about once every couple hours and if you miss it taxis can be expensive or hard to find. It took me a little under three hours to reach the beach and around 30 euro. The return is the same story. So six hours of transportation - a bit much for a beach day. By comparison Ostia is about an hour and 1.50 euro away. Both have their strengths so I'm glad I have such a long summer ahead of me to make some (very scientific ;) comparisons.