I have tried downgrading before but unsuccessful. I did factory restore, download the firmware of 5.0 and manually restore to this firmware, many errors came up in iTunes.
And i finally found out that i need SHSH blob. This lesson taught me to back up my SHSH before upgrading to newer firmware.
But anyway, next time, if you are unsure, read through forums, or surf on the internet first before upgrading. As downgrading is troublesome and it takes a risk of having to restore your device. 🙂🙂