I feel the need to add my own experience here - today I finally, after 1 year of struggling removed MetroMail from my phone and started using the native Outlook app on the phone.
My notes on why I think this app is not usable:
1. It is completely unreliable. Notifications work - until they don't. And then you don't know why they stopped working. Tried resetting and nothing. Who wants an email app that you are not sure it's notifying you correctly at all??
2. If you remove and re-install it - it will bug you to buy it again. Simple closing and re-opening of the app makes this go away - but I learned this the hard way - by paying for it twice.
3. Support sucks - no, sorry - there is NONE whatsoever. I mean common. This is one of the most important things for most people. We don't expect phone support but replying to people with some *serious* issues and bugs is mega important. At least forum could be created by the developer. But hey - they don't even have a website! :-|
4. Other bugs, like settings not saving and keep bugging you or resetting after some time. Again, how hard can this be? It just needs to be resolved once. Why lose customers over it.
Now, I understand this is an app from a one-person developer, but this means nothing to the end user. And I understand him - for a while. After a year, it's just too much. Make it open-source if you can't handle it. Or make it (ad)free and continue with the "I don't have time for this" attitude - it would be much more understandable for an app you don't have to buy.
Lastly, of course, I would never write so much here if I didn't *really* like the features of MetroMail because they are the best ones in any current WP mail app - hands down... And this is why I stuck around with it for so long - when the app is working I love it.
So... I just hope Windows 10 Mobile Outlook app will be even better and it looks like it could be. I certainly hope so because I think lack of a seriously awesome Mail App is a MAJOR turn off for many people that would think about using WP otherwise.