I managed to visit 3 different Giraffe's when I was in London: Southbank Center, Russell Square (which is not called Russell Square but is near there), and Heathrow.
It's a cute little cafe type of restaurant with a long menu of English food - I didn't have any great meals there, the menu had a lot of foods that just didn't appeal to me. The food itself was good, but not great. I also didn't get to see Giddy. But the locations were great - Southbank center is a short walk from the London Eye and in a nice shopping/eating mecca where you could tell it was more of a local place then a tourist place. The one near Russell Square was in a pedestrian mall with other restaurants and shops - another great location that doesn't scream tourist.
Overall, I wouldn't suggest anyone run there, I would check out the menu and if you see something that intersts you, then go. If nothing interests you on the menu, go to a pub and get fish and chips. But if you love giraffes, like me, then go, if nothing else the theme is fun.