Too much trial and error and a lack of in-match unit info hamper an outstanding, original game.
From the minute you enter the game, the quality and level of detail is apparent. There are numerous in-game elements that make the package feel complete, and great value considering the $30 price tag.
I won't explain the puzzle premise because Gamespot's review has already done an excellent job. I will, however, elaborate on the points that they made in regards to trial and error. There is nothing fun about entering a fight without any idea if you're ready for it or not. You find out at the loading screen. Often, you will find that the enemy units are substantially stronger than your own, leaving you with a choice between fighting (and usually losing) an uphill battle, or you turn off your DS and 40 seconds later you're back to your previous save.
Once you are actually in-game, there are basic stats that you will need to memorize because there is ZERO unit information available while you are fighting a battle. Putting your cursor over a unit gives no details - you won't know how many turns it takes for a unit to charge, how many hit points it has, or how much damage it will do, until you actually try. Furthermore, you NEVER get to see how strong specific enemy units are.
The leveling curve, while relatively quick, often seems to leave your elite and champion units underleveled compared to your enemies. In all 3 chapters I have played so far, my elites and champions (the BIG units) never reached L5, and I was constantly fighting L5s, even in earlier fights. They simply do too much damage to deal with. It's frustrating, and often means imbalance and the need to fight the same battle again.
There is also too much random luck, and too much punishment for bad luck. Your champions and elites often spawn in very inconvenient locations. You have two choices - you can either move units around over multiple turns to improve their position (which often means letting the enemy get the upper hand) or you can remove the unit from the board. Unfortunately, removing the unit from the board will cause you to LOSE that unit from your arsenal, forcing you to buy more, and they are NOT cheap - a single champion unit will cost you more than you win in a single battle.
You can also lose your champion/elite units if they are struck in battle before you manage to stack them into attack formation. At the same time, your AI opponent doesn't have an arsenal to maintain, so they don't have to waste turns moving units around defensively.
If I could make two changes to the game, I would remove the unit loss and penalties for losing a battle, and add some in-battle information.
The game is nevertheless balanced (assuming you get into a fight against enemies your level, the 5 races are well balanced) and fun. The puzzles are intriguing and rewarding. Interestingly, if you quit a puzzle mid-way you get penalized in gold, but if you fail and opt not to try again, there is no penalty.
The music is very decent, and reminds me of Puzzle Quest. The 2d graphics are clean and well presented. The story is reasonably well written and interesting enough to keep you moving from one area to the next.
Despite it's flaws, Clash's unique battle puzzle system is fun and entertaining, and the title provides a lot of hours of gameplay for your $30.