You cannot revert (which means ‘to go back’) back.
The answer to whether you should revert to OR go back to using DH isn’t straightforward. If you want (or need) a highly reliable email solution then relying on a shared web hosting provider may not be your best option. If you are not dependent on email then you have little to lose.
DreahHost needs to get rid of “This is our new mail interface! We love it, you will too!” This ruins the web interface, and the change is now months old. And DH is not reverting to Squirrelmail or RoundCube.
Others have commented on features Atmail is lacking – such as multiple profiles, the ability to customise the logo. But if these are not important to you, then I find that Atmail does work. Albeit, it is clunky.