I highly recommend taking this trip up the Bosphorus! On a beautiful day, it is a nice change of pace from the city sights.

A lot of information has already been provided by the previous two reviewers of the Bosphorus ferry ride so I will just add a few tips of my own:

*Definitely get there early (at least 1 hour before departure) if you want to get a good seat.

*As of yesterday, the roundtrip price for the journey was 12.50 Turkish lira.

*I recommend sitting on the port/left side of the ferry when you go up the Bosphorus and when you come back - this way you will get to see both shores but you will also have advantageous light for photographic purposes (in the morning when the ferry departs, you will have the sun behind you if you photograph off the port side and the same will be true in the afternoon). Conversely, if you aren't keen on photography and want to sit in the sun, you should sit on the starboard/right side of the ferry.

*I also recommend doing the first leg of your journey in one of the seats on top of the ferry and the return sitting on the seats close to the water for variation's sake.

*Definitely bring a light sweater or fleece - even in the sun, the wind can pick up and you can get quite cool!

*I am posting a picture of the location where you buy your tickets (there are several such booths along the quay but if you find this one, you should be good to go!). It is the ticket booth closest to the bridge.

*I got off in Anadolu Kavagi and stayed for the three hours before catching the 3pm ferry back to Istanbul. A lot of guides recommend the fish restaurants in the town. However, it is worth noting that there is a cafe/restaurant as you approach the fortress - it has a great view and I almost decided to lunch there myself (n.b. I am not sure if seasonality would affect opening times). Also, a lot of locals who were on the ferry and who went up to the fortress seemed to have brought their own sandwiches/picnics. I think if I went again, I would do this rather than go to one of the fish restaurants in town (what I ultimately decided to do). It was really beautiful up by the fortress and I could definitely have spent more time up there reading my book and relaxing outdoors!

*Before going, I was worried that a six hour trip would be too much but in fact it was just perfect.

Take the public ferry from Eminou Terminal 3 (down near Golata Bridge) and cruise along the Bosphorus. 90 minutes of sheer delight to the end of this busy channel stopping on the Asian side wher eyou must climb to the castle for great views before lunching back at the water front...and then 90 stunning minutes cruising back to the Golden Horn...best value in town at justover £5 per head...fantastic...and don't forget your camera.

The Bosphorus Tour ferry sails north from Istanbul, up the Bosphorus, almost to the Black Sea, then back again. Along the way it stops at the small ferry ports that dot the coast, letting you get off at whichever suburbs and villages you care to visit. The landscape you see is wooded hills, dilapidated waterside estates out of a different era, sleek modern estates very much of the present, and the occasional palace and castle.

A random anecdote:
At some point men who really want to sell you yogurt board the boat. "Yogurt yogurt yogurt yogurt yogurt yogurt yogurt yogurt yogurt!" they holler and repeat. They are very excited. Their every movement and expression insists that you will be less of a person if you do not try their yogurt. A friend explained to me that the neighborhood from which these sellers hail (I think it is called Kanlica) is known for its yogurt. Mystery solved. (I tried a spoonful of someone else's yogurt. Guess what? Plain yogurt.)

Some more concrete details:
* You can get off at any of the stops, and they say you can get on the next boat with the same ticket. But during the off-season (before June, I believe) there is only one boat a day! (During the on-season, summer, there are three.) So if you do get off, you'll have to take a bus, or a separate ferry, back to town.
* The complete trip takes SIX hours! Off-season, the one ferry departs from Eminonu (Istanbul's ferry-hub neighborhood) at 10:35 and comes back around 4:30. On-season, the last ferry departs at 1:35, with a planned return at 6:30.
* This tour departs from Terminal 3 in Eminonu. It's easy to find, by all the other ferries. (There seem to be a few other, newer tour operators that try to steal this tour's business. I can't say how their service is, but their ride is much shorter.)
* Arrive *early*. For example, get there by 9:45 for the 10:35 ferry. The boat is large, but there is not much outdoor seating. If you get there at 10:15 or 10:30, you may find the entire deck is already full.
* If you stick with the ferry ride to the end, you are let off at a town called Anadolu Kavagi (silent g), which has a steep hill you can climb to get to an old Byzantine fortress, from which you have stellar views of the Bosphorus to the south and the Black Sea to the north. No entrance charge.
* At this end stop you have time for lunch. There are supposedly excellent fish restaurants in this village. The guide book I had recommended any of the eateries crowding the port, but the one I went to had so-so food and looked like it catered only to tourists. I ended up with a feeling that a smaller restaurant with a less glitzy location may have been better.

Overall, the Bosphorus Tour from Terminal 3 is a really smooth ride, with beautiful scenery. The fortress and views at the last stop are definitely a highlight, and you can get a quiet fresh-fish lunch by the water. However the whole thing takes 6 hours, and after a an hour or two of sailing by cozy water-side villages... well, it may just be too much of a good thing.