We do it every year it's that good and great value for money for what you get to do! In fact, we're doing it again this July.
The tour basically consists of going around the Serengeti in a roofless truck with your tour leader and you get to see lots of different animals before getting to the giraffes including zebras, rhinos, antelopes etc, which your tour leader will tell you more about as you're going around. There's also a photographer on board with you who takes photos of the animals on your tour, which will be included on the photo cd if you buy the full photo package of your tour.
Once you get to the giraffes, everyone will have the opportunity to feed and possibly touch the giraffes depending on how close they come over. One word advice - watch out for dribble when feeding them. It's not pleasant getting it on your head! The tour leader keeps hold of the food in a box and only gives you one piece at a time to make it fair for everyone and they also make sure that everyone gets to have lots of interaction with them (unless you get a selfish family that tries to hog the giraffes and won't listen like we did a couple of years ago). At the end everyone's family/group gets to take posed photos with the giraffes whilst feeding them, which you have the option of buying after the tour.
A couple of things to keep in mind: all loose items and bags have to be put into a cupboard/locker that your tour leader has the key to before getting on the truck - cameras and water bottles are allowed but you HAVE to keep hold of them at all times, depending on the time of year you're doing it I would suggest doing the very first tour in the morning because later on in the day it's either too hot or it could get rained off which you don't want, also stay hydrated whilst doing it because there's no shade and it can also be incredibly bumpy because there's no seats in the truck and you have to stand and hold on to the side of the truck when driving.
Hope that helps. If you have anymore questions feel free to ask.