Quite fun, very challenging to reach a high score, but critically flawed by the fact that some target combinations are unavailable - this is worse if you have used a couple of large chains recently, and have been left with only a few tiles to manipulate. A faster regeneration rate and targets that are guaranteed to be feasible would make the game less frustrating and improve a player's sense that their score is meaningful (i.e. depends more on the skill of the player than on luck over choice of targets).
Most of the time there are multiple possible chains to reach a given target. Perhaps in order to give the game a sense of difficulty progression, it should evolve from giving targets which have been checked to guarantee many possible chains (e.g. at the beginning, perhaps the game should try find a target value with at least 5 possible chains), to ones for which fewer possible chains have been checked (so although in some cases there might be 5 chains, the computer's only checked to guarantee there are e.g. two). This would be a better method of increasing the difficulty than simply increasing the size of the target number, since at present not all the target numbers are achievable.