、。
Frankly, I think he's a good boss.
Literal
Frankly speaking, he [topic-は] good boss is [quotative-と] think.
率直に言うと (to speak frankly) — the と-conditional flavor of the frank-speaking idiom. Worth noting that 率直に can preface praise just as easily as criticism: honesty cuts both ways. と思う frames the judgment as the speaker's personal assessment.