I used a Kobo Mini and XCSoar last season. Its refresh rate is about the same as a PNA but slower than an Android phone. What is 'distinctive' about the e-ink screen is the ghosting you get - there is always a slight shadow of the previous image on the screen. Over time, with enough refreshes, this may disappear but sometimes it's necessary to skip to a (largely white) menu and back just to clear the ghosting. Not ideal but the sunlight readability more than makes up for it.
I've made up a few Kobo Mini / XCSoar /GPS bundles and also found that the screen quality, in terms of its ability to hold high-contrast text without refreshing, is quite variable. I'm hoping that this will improve with further production volumes or bigger screens. Such are the joys of early adoption.