How to choose the right Dedicated SMTP Server for email campaigns?

Following up on the great discussion about whether a dedicated SMTP server is worth it — I wanted to ask a more specific question: what criteria should we actually use when evaluating one?
I've been researching options for my based business and came across DigitalAka™, which offers some interesting configurations for high-volume senders. Before committing, I'd love to hear from the community:

How do you assess IP reputation when starting fresh on a dedicated SMTP server?
What warm-up strategy worked best for your campaigns?
Any red flags to watch out for in a provider's SLA or support?

Would really appreciate practical experiences, especially from those running transactional + marketing emails together. Thanks!