Portable turn based strategy done right.
In point of fact, Advance Wars 2 isn't much to look at. Sprites are small and lack detail. Backgrounds are simple and there's not much variance between it's many levels. There are short cut scenes for attack sequences w/ superior graphics but these grow tedious w/ time and you can thankfully turn them off. The music is only so-so as well, mostly consisting of synthesized rock tunes. These are relatively catchy but really just not up to being listened to for the dozens of hours you inevitably wind up sinking into this fine game so it's good that you can turn these off as well. What you are left with is pretty much pure strategy and that is all you really need as this proves.
The plot revolves around the many colored nations of the world being attacked by the villainous Black Hole Army. There isn't much subtlety here. The Black Hole guys are just vile and you have to beat them. Some of the characters you will like to a greater or lesser degree but that is mostly inconsequential. The characters, almost all of whom are playable commanding officers otherwise known as COs, may have personalities but they are only important insofar as they relate to that COs special abilities.
This is where the gameplay starts getting deep. Advance Wars 2 is, at its core, a basic turn based military strategy game. You get a couple times of foot soldiers, a few tanks, planes, artillery, helicopters--standard stuff. The troops for each of the many nations you play as are basically the same but the COs are quite varied. One has great infantry units and weak tanks. Another has good helicopters. Another gets troops at a discounted rate. On most of the more difficult, you can chose which CO you use and there is just as often more than one particular strategy you can carry out successfully. Beyond just the relative strengths and weaknesses as commanders, each CO has a couple special abilities that are charged up throughout gameplay. Initially, these feel almost like a needless detraction but as the game's difficulty increases, using these abilities to their fullest become critical to winning. The end result is that in choosing your CO, you wind up making critical strategic decisions before the battle has even begun--not that it ends there.
There's a few basic mission types in Advance Wars 2. There are missions where you start w/ few troops, build up a conquering army, and eliminate the enemy entirely; missions where you start w/ a large army where you must survive or defend something for a set number of rounds; and missions where you have an objective to destroy one or more enemy installations in particular. There are, of course, also missions that combine two or more of these objectives. Knowing the strengths and limitations of your various troops is absolutely critical. There are very few missions where you can get away w/ cranking out the strongest level troops and rolling over your enemy. At minimum, you need to exploit what advantage you have initially and then start cranking out troops and for most missions you need to be on your toes the whole time, especially if you want to wind up w/ a top score at the end of the campaign.
The scoring system is actually where a great deal of this games difficulty can be found if you wish to seek it out. Beating the campaign w/o worrying about score is definitely tough but getting perfect scores on each mission can be pretty brutal. Making a small error at the beginning of a mission can completely ruin your chances. You can restart the mission if it's not going well but you can't go back and replay the missions once the campaign is finished. In other words, if you want a perfect score on the whole game, you gotta play it perfectly start to finish w/ no chance of do-overs if you come up close. It's tough but it's optional and it's all the more satisfying because of this.
This lends a good deal of replayability in its own right but there's also an additional hard campaign that is unlocked when you beat the normal one and the War Room missions. The War Room missions don't have quite the subtlety of the main missions--they are universally of the build up your army and eliminate the enemy as fast as possible variety--but some of them are quite challenging and they provide for a nice change of pace if you just can't put the game down but are temporarily frustrated on your main missions. There are also a Vs. mode where you can play against a CPU or a friend and a design mode where you can pick color schemes for your COs and build custom missions. These extra features could merit a lengthy review of their own but for the sake of brevity we'll just say they are a very welcome addition here and will add hours upon hours of gameplay for those who just can't get enough.
If you are the kind of gamer who sometimes tires of constant fast button pushing and wants a more cerebral experience, Advance Wars 2 is definitely going to be right up your alley. Even gamers who sometimes find turn-based strategy to be obtuse might find the simplicity of gameplay combined w/ the depth of strategy to be unavoidably addictive. The turn-based format works great for the GBA too. You can save at any point and pick up from there whenever you have a free moment--and you'll probably find you'll be making some to get a few extra minutes on this one here and there.