Sara - speaking as a confirmed Paxos addict, I'd say it's definitely worth a day trip. It will mean a very early start, as the hydrofoil leaves Corfu at 7am (Tuesday-Saturday), but this will get you into Gaios for 8am, giving you plenty of time to explore the island. Avoid Wednesdays, since there's no return hydrofoil!
My suggested itinerary for the day would be as follows:-
Arrive Gaios 8am. Have breakfast in the main square and a wander around the harbourfront area, then catch the 10 o'clock bus to Loggos.
In Loggos, either head for the nearest beach (Levrechio) or have a walk up to the old mill for fantastic views of Loggos harbour, then catch the 11.50 bus into Lakka.
Explore Lakka and have a leisurely lunch in one of the many harbourside tavernas or cafes before catching the 2.15 bus back to Gaios. This will get you back to Gaios by about 2.45, giving you an hour or so to have a look around town before catching the return hydrofoil to Corfu at 4pm.
Be sure to check the bus timetable on arrival on Paxos. The one I used is a couple of years old and although it seems to have stayed the same for the last 12 years, you never know!
Hope this helps.