Some may miss the 'cute warfare' feel of the last 3 games, but Days of Ruin is one of if not the best in the series.
The campaign is hard, especially on the later levels, but it is possible if you have some patience, though those with a short temper should probably stay away--some of the levels are pretty frustrating, but this just makes it all the more satisfying when you finally beat the game. The storyline is far more mature than the last few games and more enjoyable and involving. The War Room has been lost (which is a bit of a bad thing) and has been replaced with numerous 'Training Missions which you unlock throughout the campaign. There's also dozens of skirmish maps that will keep you and some willing victims busy for some time, and you can even design your own maps (which is fun in and of itself, let me tell you!). The multiplayer is a LOT better now that WiFi is available, but multi-card play and pass-the-DS games are still just as enjoyable.
Players of Advance Wars 1 through 3 are also going to have to get used to having greatly weakened CO powers which makes this game work much, much differently. On the positive side, though, your CO can actually get out there and cause some hell now, which is a very-much welcome addition to the game and helps offset the reduced CO abilities. Your units also gain experience now, which makes target selection more interesting… should I go for the full-health rookie tank, or the weakened veteran one?
The biggest negative is the loss of the Battle Maps shop, which makes unlocking new COs and maps far less satisfying. There also aren't many AI difficulty settings (minor detail, really, because the standard AI is quite good) and not all that many Cos (and they could have added a few more quiet easily… we'll just have to wait for Advance Wars 5, I suppose). The units are also much closer together in terms of health and firepower, so there won't be anymore Neo-Tank rampages (which is good and bad, depending on which side of the rampage you were usually on…)
Most of the negatives are superficial, though, and the unit and CO changes help to set this game apart from the last three. After all, if you want more powerful COs and units, just go grab Advance Wars 2 or Dual Strike and start playing. The loss of the War Room is offset by the WiFi connectivity (which is AWESOME) and the very-satisfying campaign.
All-in-all, Days of Ruin is a fresh and worthy addition to the series and is definitely worth getting, whether you're an Advance Wars fan or not.