I like the SumSum idea.
Every so often I need a new type of Sudoku- or Kakuro-type puzzle to try out. These go into fresh territory, and avoid the boredom of the same old techniques and same old trial and error.
A new puzzle type, though, should not be excessively difficult. It's more fun to discover fresh solving techniques than to go through the drudgery of lengthy trial-and-error sequences, or to try to come up with advanced solving techniques.
In the case of SumSum, I started with the easiest (5-digit), and was fascinated by the techniques I discovered. They took a while, but not too long. The 5-, 6-, 7-, and 8-digit puzzles (the initial ones posted back in April) were about the right degree of difficulty. Then I got stuck on the 9-digit one. After figuring out where most of the black squares had to be (the most fun part of SumSum), I could not resolve which two of the four cells r5c5, r5c6, r6c5, r6c6 had to be black squares, so I eventually gave up and haven't been back.
So, if a new puzzle type could stay in the moderate range of difficulty, you'd keep me as a "customer" longer. Once a puzzle becomes fiendish, I'm no longer interested. The fun in new puzzle types is in discovering the basic solving techniques.