My biggest complaint with my Zumo (660) is the poor design for plugging it in to charge, or connect to the computer (which apparently doesn't charge, or not well). Had some growing pains before my trip with getting it charged.
Was also extremely frustrated that it had limited custom routes, especially when I got more for STAR than could fit. I ended up storing them on the card, and then importing them (and deleting others) as needed.
Some learning issues along the way, turn off MC mode, turn off recalculate, etc. and things worked better.
One observation on "time to arrival" calculations, I believe these are done based on some average over the lifetime or since some data reset of each particular GPS. Was traveling with my brother, using either MSTA routes for STAR or ones he created, and my time was often shorter than his. Then I realized I rode 200+ miles to his house on highway cruising speed, much higher than we ride on back roads, so my average was higher making my arrival times shorter. Many times we thought the routing was actually different.
Great tools, but we still need to think for ourselves, learn and adapt, and have some patience with new things (not my strong suit).