你倒别说， En, it's true... Yep, indeed,...

你不用怀疑 / 不相信，（事实上）还真是这样。--- you have to mention,it's really in that case.我理解是这样的，不知道准不准确。:)

这是非常地道的中文，通常后面连着“还真是这样'。一般在聊天有一点小分歧时出现。 Which is very native chinese,normally,there is a sentense"还真是这样".always appear when there is alittle divergence in talking.

你倒别说...... means that something is beyond your expectation. In other words, it's normally used for "good case". for example, 你倒别说，这里还真美。 ----- it means that you know this is a beautiful place but when you see it, it's more beautiful than what you imagine. There is an english expression: You don't say! they are similar.

补充一下，一般应用在别人请你欣赏某件物品、参观某个地方、观赏某次演出，或者是独自发现这些东西的时候，当你想要赞美他(它)们的时候，就会说“你还/倒别说，....”。