Haven't started the Mercury puzzle yet, but apparently had to rate it in order to comment on other puzzles. The Sun puzzle has been the most challenging so far and I almost gave up 2/3s the way through due to the multiple shades of red.
Earth was easier with the Moon falling in between. The quality of the pieces helps - they either fit or they don't - as does the detail of the enclosed poster; the latter proved the key to the Starfield puzzle.
Looking forward to Mercury and Mars with the new...