I like that it's easy to set up with multiple domains/aliases so you don't have one account everywhere! It also has great reporting features which makes managing everything easier than ever before (and if not there yet - they're working hard!).
The pricing model could be better explained in detail at signup but other then this minor issue nothing really stands out as bad about authSmtP itself. We use them because of their ease of setup & configuration options along side our own domain management system we built ourselves using Zendesk tickets / emails etc..