Dungeon Explorer is kind of sloppy, to even be able to play multiplayer you have to beat the entire game first...
Not true. You get access to multi-player pretty early on in the game. iirc, it's right after the point when you acquire a school. Did you actually play the wi-fi co-op or are you just passing impressions along?
I hate to say this but Dungeon Explorer is nothing but shovelware, seriously, look elsewhere before trying DE.
Playing a half-decent adventure RPG like Eragon ALONE is way better than playing Dungeon Explorer with 2 other people right next to you.
Just because you don't enjoy the type of game, doesn't mean it's shovelware. I think it's the best dungeon crawler on the DS right now, and I've played Izuna and CoM all the way through. And BTW, I really enjoyed Eragon for the short time it took me to play it, but DE is nothing like Eragon. One is a dungeon crawler, and one is an Action/Adventure RPG. If it's not your cup of tea, fine, but there's no reason to trash a game just because you didn't like it.
I've played DE co-op quite a bit now, and I think it's very fun. You can co-op with friends or with anyone who's looking for a game. Not to mention it gives you access to the best items in the game, an you can beat the final final boss with help from a couple friends.
Just the fact that DE has been able to successfully pull off wi-fi co-op is impressive to me. As you can see from this thread, it isn't easy to do.