At 4 months dd would roll the lenght of the room to get to something that she wanted. So I never thought they would be useful. If you have one of those nice, helpful, kids who sit where you put them (and I never managed to get one of those!) they might be useful
I got one with silver lining from Ebay for about £15. It was a complete godsend the year dd was tiny. We could still spend the day on the beach, didn't need to lug a buggy down there, she was protected from the sun, and happily slept on a rug in there, and I could sit in it and feed her.