I’m only using it in the simplest way (transactional emails, no special formatting), and so far so good. They have really good support and documentation and the plugin from @vini_brito makes connecting it to Bubble easy for a non-technical person.
As you know Bubble has built direct integrations with SendGrid so that’s the easiest to use if you want emails to use your domain name (if you search SendGrid on the forum there are some long threads about it and you can make your own determination). If you go with Postmark or any other service besides SendGrid, you’ll need to recreate the emails Bubble generates for email confirmation (if you want that) and password resets. The latter is easy but the former takes a little more work and it’s not as clean a process as Bubble’s built-in process via SendGrid. If you search “Postmark” on the forum there’s a thread where instructions on how to do that are discussed.
Separate from this, with any third party email service sending using your domain name, there are settings at the domain level you’ll need to implement to help those emails get recognized as valid by the recipient. The email provider you use will have instructions on that.