Yes, it's a nice day out if you've not been before and takes roughly 2 hours if i remember correctly.
The boat takes you firstly to a monastery, Panormitis for a quick look around then on to Symi Town itself. Plenty of time there for you to take lunch and have a wander around the town.
You can book through your Rep but being so close to Rhodes Town it's as easy (and cheaper) to get a taxi and buy your tickets at Mandraki Harbour.

Yes we did it from the harbour last year, it is cheaper.

When you get off the ferry in Panormitis, turn left and follow your nose to the bakery which is run by the monks. Chocolate croissants to die for!!!

You don't say when you are going but don't make the mistake of taking a seat outside on the top deck of the boat in May early June as you will freeze to death as soon as the boat picks up speed. It's a great day out to Simi but the harbour town gets very hot at midday. As I remember you have about three hours to spend before the boat heads back. I think the fare was 30 euros for two people if you book your seats by yourself. Make sure you book in advance.