For versions lower than iOS 7 use the old one:
itms-apps://itunes.apple.com/WebObjects/MZStore.woa/wa/viewContentsUserReviews?type=Purple+Software&id=YOUR_APP_ID
This works on my end (Xcode 5 – iOS 7 – Device!):
itms-apps://itunes.apple.com/app/idYOUR_APP_ID
itms-apps://itunes.apple.com/WebObjects/MZStore.woa/wa/viewContentsUserReviews?id=YOUR_APP_ID&onlyLatestVersion=true&pageNumber=0&sortOrdering=1&type=Purple+Software
Code snippet (you can just copy & paste it):
#define YOUR_APP_STORE_ID 545174222 //Change this one to your ID
static NSString *const iOS7AppStoreURLFormat = @"itms-apps://itunes.apple.com/app/id%d";
static NSString *const iOSAppStoreURLFormat = @"itms-apps://itunes.apple.com/WebObjects/MZStore.woa/wa/viewContentsUserReviews?type=Purple+Software&id=%d";
[NSURL URLWithString:[NSString stringWithFormat:([[UIDevice currentDevice].systemVersion floatValue] >= 7.0f)? iOS7AppStoreURLFormat: iOSAppStoreURLFormat, YOUR_APP_STORE_ID]]; // Would contain the right link