率直に言って is a similar phrase meaning 'frankly speaking', but 正直言って is more common in everyday conversation and carries a slightly softer, more personal tone.
Standard kanji-kana spelling for this fixed expression.
Kana for 正直 is common in casual writing; the expression remains the same.
A fixed expression combining 正直 (honest) and the te-form of 言う (to say), literally 'saying honestly'.