There are some "Howevers."
His documents don't back up much of what he says. There isn't a lot of confirmation yet to much of what he says. He is an awful guy who was at the center of this plot to extort the president of Ukraine for months. He's never shown remorse for that, only saying he wants to set the record straight. And he's an awful guy from his past history prior to all this, as well.
To be clear, none of the "howevers" mean the reasons I believe much of it aren't valid. Just that I don't begin to see him as this heroic, whistleblowing character, and he's a very untrustworthy guy. I simply think that it has to be investigated further and verified. But it's certainly an extremely strong starting point.
The biggest starting point is that it's near-impossible to believe that he did this without full support of Rudy Giuliani, and that Rudy did this all on his own without support of Trump, and that Trump kept all of this hush-hush quiet without protecting himself by those around him he relied on. And the Parnas interview and papers do support all that. So, that's in large part why I do believe most of it. I just kick in with the howevers and would love to see this serve as the start of a full investigation.
By the way, on a separate matter, though Trump-impeachment related --
As we all know, Trump has a standard way of operating which is to distract when bad news is coming his way. So, I feel near-certain that when it was known the House would be signing the Articles of Impeachment today, Trump (being Trump) thought it would be a great idea to distract by announcing the apparent signing of the China trade deal he's been trying to tout for SO long. But in what universe did he think that pretty much any news would top the signing of only the third impeachment in U.S. history???? Forgetting for the moment that the "trade deal" got generally ridiculed. What the announcement mainly got was buried and lost in the news. So, this loooong build-up that Trump has been hyping for what seems like forever, on which he's likely been expecting massive praise and hosannahs...was totally, utterly wasted.
The heart bleeds.