With each version that is released, SmarterMail 16.x gets better, just as SmarterMail itself has since it was initially released. There is always some confusion and panic and "revolt" when things change. We've experienced this every time we've changed the interface or added or modified a feature. However, each change is built upon and improved, and the initial confusion eventually subsides.
As for using an external client, that's certainly an option. Outlook, Thunderbird, Apple Mail, eM Client all work well with SmarterMail over IMAP, EWS, or even using a webDAV protocol. We recently worked with the guys at CalDAV Synchronizer to get a fix put in place that greatly improves their product, and now it works as well, if not better, than our Add to Oulook plug-in.
The .1 version that will be released this week brings even more improvements as well as the return of some features present in previous versions. A few examples: the ability to move the preview pane (or remove it completely); the ability to select contact lists, categories and contacts from the new message window; the ability to load or block images in the webmail client and create exceptions for allowing remote content by user/domain/URL, and more. That's on top of resolutions to many of the issues brought up in this Community as well as in tickets we've received.
There's no doubt that SmarterMail 16.x is a huge change...and it could be argued it's the biggest change we've made in the last 15 years. However, we're committed to it and committed to continually improving it. Obviously the larger issues are handled first, then we'll work our way down our list as quickly and as efficiently as we can, updating Community threads along the way.