Should you level up to save Tokyo? I don't see why not.
I bought a DS in the fall of 2009 to play Scribblenauts, but it turned out that I've spent most of my time on the system since then playing Shin Megami Tensei: Devil Survivor. It's a great game that is easy to play in either long or short bursts, making it perfect for a handheld system.
Like so many others on this site, I had my interest in the Shin Megami Tensei series piqued by the Persona 4 Endurance Run. I had very little experience playing RPGs before playing Devil Survivor, unless you count Mass Effect. So while I'm not an expert on the genre by any means, I'm comfortable saying that this is a great RPG that doesn't suffer from being a menu-based game.
I won't discuss the specifics of the story much, but it suffices to say that you're a teenager and you and your friends are suddenly burdened with the task of saving a locked-down Tokyo from demons in seven days or less. I thought the story moved very well and kept me interested and the localization team did a near-perfect job translating it to English. The characters aren't all amazing, but they're varied and easy enough to care about or--at least--be interested in.
The game breaks up the days into 30-minute chunks which are spent fighting demons or getting information from other people inside the lockdown. You can grind out some EXP without advancing time, which is important. Playing these Free Battle events are essential to completing the game, but--because it's a handheld game--you'll often find yourself with a nice time slot available in your day to comfortably play a Free Battle or two. They usually don't last more than a few minutes.
I was fine with the difficulty of the game until the last hour or so. I was having very little trouble with battles until the end of the game, when I was very underleveled for the first time. This seems like a mistake rather than a decision, as it really hindered the flow of my experience. Nearly everything involved in playing the endgame felt unnecessarily time-consuming and difficult to me. I had to spend a lot of time leveling up to even have a chance at beating the bosses I encountered at the end of the story.
The game feels pretty intuitive, but there are certain things that just aren't explained very well or at all. I didn't realize until I was grinding EXP to fight the final boss that you can press Y at your Team page to assign skills to yourself or your allies. I thought you had to learn the skills naturally and the fact that something so important wasn't very clear is pretty annoying. There are also some status effects that aren't explained very well. After finishing the game, I couldn't tell you what being Paralyzed actually does. You can still attack, use MP skills, and move after being Paralyzed. The only consequence I saw was that there's a high-level MP skill that instantly kills any person or demon who is paralyzed.
Just like in the Persona series, there is demon fusion in Devil Survivor. It's much better than the system in Persona 4, as you can see a list of all of the demons you can make at your current level as well as the ingredients required to make said demons. It makes what could be a very tedious process very easy to handle.
The graphics seem fine. They're no technical achievement, but they're well-done for the style in which they inhabit.
The sound, however, is a problem for me. I played most of the game with the sound off. The soundtrack is mostly a hard rock/heavy metal kind of an affair. The songs (which are most commonly pretty short loops) in the game are okay-ish the first time, but the music is extremely repetitive and, because it's often distorted guitar at a not-so-great audio resolution, it becomes irritating fast. There's no recorded dialogue in the game, which shouldn't be a disappointment, but is worth mentioning.
Despite an irritating soundtrack and a level of difficulty that accelerates significantly towards the end of the game, if you have a DS and a taste for RPGs, then I don't think you should miss this game.