I used the auto guide on my round Britain last autumn. I would say its fantastic, when you are in unfamiliar territory, things are getting a bit rough, there is no option to pull out a chart, the auto guide comes into its own and really helps make a sound decision to change course for a friendly port which wasn't on your original passage plan. We didnt use it much, I would say about 8 times, but each time it choose the course we would have expected, very quickly and easily. Really good to know its there.
I never found the garmin Blue Chart tide calculations accurate, so we would always take a safer option and make sure there was more than enough depth.