Generally, there are 3 areas why I like working with ExpertSender: Reporting/API, Targeting and personal support.
1) Working as a Data Scientist for an Email Marketing Company, detailed insights into our performance as well as an effective system to make sure that our messages reach the right audience are key. Fortunately, Expertsender offers one of the best reporting systems, in terms of depth and UX, I've encountered in an ESP so far. Statistics can be aggregated in multiple ways from simple date ranges, over domains and domain families (extremely helpful for inbox providers that have multiple domains like Microsoft) up to the individual IPs that were used during the send out. Also available inside of the platform itself are external statistics like SNDS or (Market dependent) Signal Spam. Nearly every report, as well as the raw user activity data, can also be accessed via API. As a result, creating your dashboards and statistics are pretty easy
2) Targeting is the reason why we looked at Expertsender is the first place (see below). Their integrated tag-system allows for easy and individual categorization. Targeting then females, interested in fashion that have been active during the past two weeks, it a) possible and b) just takes those three conditions during the segment creating.
3) Delivering large amounts of emails is more complicated than many people realize. Expertsenders deliverability and in particular our account manager have helped us to identify and fix flaws in our strategy and templates.
As an ESP and not an SMTP relay, it's currently only possible to sent a mailing via API to lists and segments that are already created in the system, instead of simply handing over an array of recipient emails - and segments can't be created via API. For a full end-to-automation, those two features are something I would wish for in the future.
Also, even though it's easy, creating more complicated segments can naturally take its time. The option to "copy-paste" parts of a segment could speed things up.
Our reason to start using Expertsender was the shift in deliverability requirements over the past years. While the spray-and-pray approach might still have worked a few years ago, many inbox providers like Gmail have, rightly in my personal opinion, have become incredibly sophisticated in keeping irrelevant content out of the users' inbox. As described under "What do you like best?", We were looking for a more targeted approach with sending individualized emails to our users, with content that is relevant to them. In doing so, we were able to increase our Inbox placement and with that Open- and Click rates.