How is 7 on older ios devices?

Dear apple core

I'm trying to upgrade a family members devices for them but I'm not sure if I should because I'm not sure how well, meaning fast or smooth, ios 7 will run on their ipad 2 or iPhone 4 since they are comfortable with the way things are. I'm sure they'll upgrade their devices soon enough, but how has your experience been in this case?

Thanks for the feedback.