Unable to keep up with the demand of Siri requests, CD-Dev Team’s servers have been pretty unresponsive, leaving the app useless at times. In addition to that, the app has been known to cause random reboots, break the camera, mess up the Settings app and cause the phone to get stuck at the Apple logo among other things.
But if you really must have Siri on your non-iPhone 4S iPhone, use the app at your own risk, or at least wait for a more stable release. And let’s not forget H1Siri makes use of Apple binaries without permission which makes it illegal as well. It is also noted that using H1Siri with your emails, SMS, Calendar, Contacts, Location etc might send them to CD-Dev Team’s servers.