It's a brave new direction for Dragon Quest. Much has been made of making this for DS, but I'm happy to report that it works well. If anything, it taps into DS's strength.
The beginning is a cliché : I started the game by customizing my main character, walked, talked and battled as you do, and it took a while to get to grips with the story and system. I was never quite sure how I should develop my character with the skill system, somewhat akin to DQ8. I stuck to the old RPG wisdom of "stick to the skill you started to develop", and narrowed down to 1 weapon skill and 1 class skill. A pretty good wisdom (to start off with, that is).
Some hours into the game, just as I begun to wonder whether I've missed the chance to form a party, I went through some events and an inn opened for business - I formed a party of 4, and the rest is history - I forgot myself as I got drawn into obsessively chasing that next big thing. I often talk about replay value in reviews. I won't this time; exhausting it the first time is long enough that you'd go truly mad to replay the lot.
The crux of the obsession for me boils down to 2 things: skill development and equipment upgrades. The main story is not the strongest I've seen, but it's intertwined with bewildering number of quests and character-development elements. The pace is set from around when you make your party, and after that you will find yourself obsessively pursuing your desires until you forget where on earth you were in the story. I actually recommend this sidetracking from the main plot. I can say that it's a great way to enjoy it, with the benefit of the hindsight.
It isn't without its faults. Developing your character means the story gets easier when you return to it, and I would say that the game could use a bit of 'balancing'. Just strengthening the monsters in the story will do. Square-Enix may have done this deliberately, though, to welcome casual gamers as they might otherwise find the story too hard.
Any misgivings I might have had on that never got in the way, as I became overwhelmed by the sheer volume of content. There is the usual "sense of journey" in this game, of trotting across large plains of grass, desert, snow and mountains, watching the classic monsters roam around freely. The world is split into large zones and the sense of scale is well expressed.
As you get to grips with the game, you would always find a reason to get skills and equipment. There's always that special something to camp and upgrade. It's obsessive enough in the single player, it gets even more so if you start to adventure with your friends in multiplay as you tend to 'compete' a bit when you have friends to show off to. I suggest sidetracking and pursue more items and skills in between the 'segments' - after forming your party, unlocking Ranger/Battlemaster, and unlocking Paladin. It's more fun that way. Multiplayer enhances the joy, but not necessary.
After the main plot more features become available, one of which is the hundreds of treasure maps. There are normal maps, and boss maps which host a dozen of the big bosses from old DQ series. Higher level maps are harder, and boss maps can be made harder with progressing levels; past level 95, some of these big bosses are mind-numbingly hard. Good luck formulating your own strategy to bring them down. There are also additional quests you can get from Square-Enix server, as well as shopping for fancy items. Another memorable feature was the hidden alchemy, which was insanely hard but extremely rewarding.
What we have yet to see is the extent to which the guest-visit system ("contact mode") will spread here. You run DQ9 in stand-by mode and carry it in your pocket or bag. When you walk within several yards of other players, you exchange a copy of your avatar, who carries the map as a gift. Your inn grows in size as you collect more guests and you get rewards for that too.
The beauty of this is that you're doing it with complete strangers you walk by. It's hard to discover all the maps on your own, so exchanging them is a great idea for DS in your pocket. Obviously, the more people walk around with this game the better this will work. I managed to easily fill 3 new guest slots every time I crossed the big road in central Tokyo, but that's because it's that popular there. We don't know yet whether that will happen here.
Alas, as all good things must come to an end, it felt like I was leaving a large chunk of me behind in the DQ world. When I put it down, over-exhausted, I noticed that single + multiplayer together I had clocked up 300+ hours. Whether that's a good thing, I'll leave it to your judgement, but this amount of content in a DS game is truly remarkable - SquareEnix surely are serious with DQ9 and it shows; this was no gimmick. Heck, it's probably one of the most full-blown games ever on DS.