There's just one big hitch...
When you (or your partners) faint in a dungeon, you lose all your money (not such a big deal) and "some of" your items. If you have a particularly difficult dungeon to get through and you faint repeatedly, your chances of making it through the dungeon get lower and lower each time, because you finally get to a point where the only items you've got are the ones you find IN THAT DUNGEON. So, you're going through this horrible dungeon, and you find TMs that you can't use, Bands to hold to boost attack (or whatever), Cheri berries and stuff, but no Oran berries, which are the only things I know of that restore HP. Walk into one Monster House room and you're doomed. I've been in the same dungeon for three days now and it doesn't look like I'll get through it anytime soon. Otherwise, a very enjoyable game for people who like closure (like me), because you can totally sweep the floor of a dungeon and make sure you've gotten every item and explored every corridor.

Mystery dungeon greatness
I honestly feel that Pokemon Mystery Dungeon is MUCH better than I expected! I mean, I thought that it would be the same as Pokemon Mystery Dungeon Blue Rescue Team, but it was better and then some! I must say that one of the best new features would be sentry duty, as it is like a minigame and is like a break from the dungeons. I wish I would have known how easy it was though. I mean, I got it a week after it came out, and four days later, I was working on the final mission before the post-ending. I feel that that isn't that a big of a deal though, and I still think that it is the best pokemon game that doesn't involve catching and training Pokemon like usually.

They got it right this time
This game is a GIANT step forward from the GB Advance version! This is the game that that one should have been. I am really not too knowledgable about the Advance game, since I put it aside as borderline unplayable and a major disappointment. The new version is vastly improved. The following areas are its strong points:

First and foremost, a good combat interface. In this version, your head Pokemon can select one of four moves, just as in the regular Pokemon games. And there is no burdensome double-button punch to use a move. You can also choose an ordinary attack, which is not a move, to save moves: a welcome Pokemon innovation.

Attractively drawn dungeons

Good and varied music

A good story line

Lots of interesting tasks and situations

The real comparison is with the recent Shiren Mystery Dungeon, also a Chun product. This one is less brutal and much larger-scale, though both are excellent games.

Highly recommended for roguelike fans.

I assume that this rating and description can also apply to the "Explorers of Time" version of the game, since the strategy guide says there are only minor differences.

Even better than the mystery dungeon blue rescue team
In this game, you play as a Pokemon and have another Pokemon as a companion. You enter dungeons with randomly-generated floor plans, collecting items and fighting Pokemon as you complete tasks. The fighting is turn-based, as with most other Pokemon games. One advantage is that you regenerate HP as you walk. However, you need to keep a supply of apples with you because you also get hungry as you walk and if you get too hungry, you faint. However, this is better than not regenerating HP until getting to a town, in regular Pokemon games.
So far, I have only played this game about 10 hours, (in comparison with over 100 hours on mystery dungeon blue team), but I prefer this newer one. Obviously, since I have played it so much, I really like the first one, so this is saying quite a lot (I am a fan of Pokemon games, in general). Here's why I like this one better:
1. you can hold more items in dungeons (and this number increases as you progress)
2. you can recruit new Pokemon even if you have 4 in your party already (and they are automatically sent out of the dungeon)
This may not seem like a lot, but these two facts alone make the game much more enjoyable. Here's the one thing I have discovered that I like less than the blue team version:
1. You see your team members' HP as a ratio without also seeing a meter-bar to give a quick visual gauge of your team members' HP (although you do get the meter for your team leader).
This is a rather small thing, but without the meters, sometimes I don't notice how low the HP of some of my team members become and so I faint from carelessness, rather than 'real' danger. Perhaps there is a setting to remedy this and I just haven't found it yet.
I will update this review after I finish story mode.