Ross-
Wow! That's a tall request. First off, I had no interest in a GPS, preferring paper maps and thus being totally involved, mentally, in navigation. That's a result of many years of land, air and nautical experience.
HOWEVER- in 2010, I realized that if I was going to lead 25 scooterists on cross country rides in Tuscany, I needed, for their sake, a way to make navigating the endless round abouts a bit easier. If it's just me and Mrs Aviator trying to sort out ambiguous signs in a round about, well, that is "quaint". With 25 riders following your lead, not so.
I had no idea of how the typical automotive GPS worked, but I had an idea of what I wasn't interested in - having to take my eyes off the road to look at a map. I saw a GPS with what appeared to be a nice, prominent "steering annunciator" and some other attractive featuures, and read further. Found a software package that allowed you to plot a course in Google Maps and transfer that to the GPS. Since I used Google Maps to fine tune the course plans to begin with, that was a plus. Then I saw that the GPS that attracted my attention was on sale factory refurbished and decided to buy it and play around with it well before the Italy trip. Now, keep in mind I live on a small Greek island, so my ability to"test" the GPS was limited. Got most of the "quirks" figured out before the trip, but not all. Fortunately, the two "burps" experienced while leading the crowd were minor and resulted in laughs rather than any major stumbling blocks.
So, some features I think are useful?
1. A nice, prominent "steering annunciator" that graphically tells you a direction change ahead and how far to that point. The bigger, the better, as it's something you want to understand in a glance.
2. I cannot say enough in favor of Bluetooth and a helmet communicator. Having Lola's sweet voice talk you through turns sure beats taking your eyes off the road.
PAUSE HERE - it also takes time to adapt your reactions and thinking to these instructions. You want to get acclimated before you are, let's say, trying to navigate urban roads.
3. The route planning functions. Some units only let you plan a route via the screen. Others allow a software interface to a computer, such as via Google Maps. If you want to make a scenic ride, you will most likely want the ability to pick the roads yourself, something which some units do not totally allow.
And of course, other features you may like, such as waterproof and "glove friendly". However, there are ways to address rain and gloves that are easy to apply if you find a unit that meets your needs on items 1 thru 3. But all the waterproofing in the world will not help you plan or execute a route.
In short, I have a $125 name brand, non waterproof, non-glove friendly, refurbished GPS that has successfully lead MV tourers twice in Tuscany and once in the Pyrenees Mountains of Spain, over a few thousand km of secondary roads, including a couple of very rainy days. That makes me a happy camper. Could a $500 - 700 "motorcycle GPS' have done a better job? Don't know. I haven't used one.
If you can, find a friend with a GPS in their car and have them let you get an intro into the subject. Hands on beats internet testimonials in my book.