Actually, my primary purpose in writing a solver is to turn it round and use it for generation: my hope is that I'll be able to invent puzzles of this type with vaguely consistent difficulty.

(There are two particularly good ones I know of: try making 24 using 3,3,8,8, and then try making 1 using 1,1,1,5. In both these cases you aren't allowed digit concatenation, and you must use all the numbers.)