I took it back and the employee said 'We open them up to test them'... RedEcksThree

What a joke. It's factory sealed, why does a brand-new game need to be opened and tested? I have no doubt that plenty of the used games they buy are re-sold as "new." 

Their used prices on DS games aren't much better, just $5 off the new price. What I really hate is when they price a popular used game at $5 less than new, but there's no box or manual. Come on!

Oh well, at least they're a good source for cheap used GBA games, there isn't much demand so the prices are usually reasonable (but yesterday at Gamestop I saw a GBA FF V cart for $25, no box/manual. Whatever!) 

I finished off the final boss at around the 42 hour mark and still managed to put another 10 hours on it afterwards, playing with different jobs and looking around to see if I missed anything (I did it without FAQs/walkthroughs). For me it was an easy game, I then went on to play the FF games available for the GBA and those were harder, I did need to consult walkthroughs a few times to check on my next step. FF III is certainly worth the money though, works out to less than a $1 per hour of entertainment.

I thought the the stories were much better in the GBA FF games (loved the unlocked stuff after the final boss in FF IV) but I still recommend FF III.

I don't know anything about those games, but Castlevania Dawn of Sorrow has been re-released as part of "Konami's Best" line and it's now $20 new -- that's a fantastic DS game. Maybe you've already played it, but thought I'd include it here as a tip for other bargain shoppers reading this thread.

You can't save inside towns/caves/buildings, you need to be outside. I always saved before I entered anything and saved again first thing when I exited.

Mario & Luigi: Partners in Time was around 20-25 hours for me, somewhere in there. I did not use any FAQS/walkthroughs. I spent a lot of time looking for beans, so I retraced steps to find beans that I missed early in the game. I found the secret shop where you can buy nifty stuff with the beans, and I wanted the most expensive item. Of course now I can't even remember what it was or what it did -- but I remember being happy that I bought it. ;)

Mario & Luigi: Partners in Time is the best, imo. It's very funny and really cute, and I don't normally go for "cute" stuff.

FF III is good, but the story is seriously lacking. I think the Final Fantasy GBA games are better. I paid full price ($40) for it and I wish I had waited for a sale.

 I want to try Lunar Knights next, I've heard good things about it. 

My recommendations:

GBA: Golden Sun, Final Fantasy IV-VI, Mario & Luigi: Superstar Saga

DS: Mario & Luigi: Partners in Time. I've also heard good things about Lunar Knights but I haven't played it yet.

I was really surprised by how much I enjoyed playing Partners in Time, it's an awesome game.

I received my DS Lite in January. That was the first time I'd played a video game in ... many years. So I didn't have a clue what was out there, I was familiar with Mario and Zelda and that's about it. Visit http://www.metacritic.com/games/ds/ and http://www.metacritic.com/games/gba. Metacritic compiles reviews from dozens of online sources and puts them on one page so it's easy to see how the critics reviewed a game. Read the reviews for games that look interesting, and then come here to Gamespot and watch the gameplay videos. I can tell by watching a few videos if I'll have fun playing a game. Everyone else may love Mario Kart, but I know that I won't, so it'd be a huge waste of $30 if I bought it based on the zillion recommendations out there. I'd rather be playing Fire Emblem, which wouldn't appeal to everyone.

This is what I've enjoyed playing the most, research and see if you'd like 'em too: Castlevania: Dawn of Sorrow, Castlevania: Portrait of Ruin, Final Fantasy III through VI (III on DS -- IV, V, and VI on GBA), Fire Emblem: Sacred Stones (GBA), Mario & Luigi: Partners in Time, Mario & Luigi: Superstar Saga (GBA), Final Fantasy Tactics Advance (GBA). I also like Golden Sun (GBA) but it got off to a REALLY slow start, so if you decide to give it a try don't give up 30 minutes in, it does get a lot better.

I have several GBA games because they're fairly cheap if you don't mind buying used games without manuals (http://www.gamefaqs.com will help you out there).