I said in my original posting that I didn't really enjoy the puzzle (because of the amount of cold solving), but I didn't say or impy that it wasn't a good puzzle. The clues are good, and some are excellent, and the theme, such is it is, is unusual and spatially interesting.
The one aspect that I didn't find particularly hard once I had a toe-hold on the grid was the entry method. Initiallly I entered al answers normally to complete the grid. After that the easiest thing to do is use a second grid and enter all the normal entries (in upper case), then rotate the grid 180 degrees and enter the rotation entries as if they were normal entries. This enables one to see how some letters in crossing entries from the other two groups appear. In the remaining cells a few letters need careful consideration, such as F, G, N, R, S.
I didn't use a mirror, but it's useful for a final check in some cases.