Potential clients often ask what are the chances that they will be successful if they appeal their case or if they have already lost their appeal, they pursue a federal habeas petition. I used to refer potential clients to an interesting Department of Justice statistical study, but the DOJ has removed the study from its webpage and I am no longer able to find it. However, the Florida Bar recently published an article summarizing statistics for the federal Eleventh Circuit Court of Appeals, and it can be found here http://www.floridabar.org/DIVCOM/JN/JNJournal01.nsf/8c9f13012b96736985256aa900624829/64e2374ed9060b9985257ae200635805!OpenDocument It says that only 5% of criminal appeals are reversed on the merits.
Of course each case is decided on its own merits and statistics are meaningless as to your individual case.