Played this game for about 6 hours straight yesterday with a couple of buddies and had a blast. We got up to the 2nd boss fight before wiping, but were able to adapt our tactics and down the 2nd boss before he got close to our crystals.
I think this game is one of the most fun co-op games I've played in years, and I don't understand where people come off saying it's repetitive, unoriginal, or that it lacks difficulty or depth. The only problems I've had with the game so far are just bug-fix related (a few disconnects and crashes), and the fact that the huntress is pretty much outclassed by every other hero.
I've clocked 21 hours into the game (a lot of it in just repeating the lower levels remaking characters on the open vs ranked servers when trendynet was being glitchy, or just restarting characters with new friends, etc), and just cleared the 2nd "act" of 3 last night. I've been most impressed with how the boss fights have turned out so far. They've all been a hectic scramble between maintaining defenses, healing, dps-ing the boss (or whatever related objective you need to kill), and making sure to replace/upgrade any faltering defenses when you get the chance.
That being said - if you are absolutely against playing co-op and want to try this game single-player, don't bother. It is a very poor single-player experience, and frankly was never meant to be played as a single-player game. Convince a friend or two to buy the game, or join a game w/ some random people online, but just don't even bother trying the game solo. It's 1/10th of what it is even with just 1 other person.
If I had to give a numbers rating, I'd give it the following:
Graphics: 8.5 - Torchlight meets tower defense. The bright visuals can be a little distracting from whats going on, but for the most part they present the action well and give the game a feel of its own.
Sound: 8.0 - There are a few good sounds in the game that you'll hear over and over (grunting orcs, screaming suicide goblins, ballistae & fire towers launching death at your foes), but I would have liked more sound effects and voice-work from the ogres, bosses, and other more unique mobs. This is not only to give them personality, but as well as to give you audio feedback to help in your defense. For example, hearing a wyvern screech or something would help give an audio heads-up before seeing "An Eternia Crystal is under attack!"
Gameplay: 4.0 for SP, 9.5 for MP - Seriously, don't bother with the single player. Sure, you can still benefit from mutliple building types, provided you have a hero of a high enough level, but this game was meant to be played in coop, and it excels at precisely that. Almost all of my time playing the game (beyond the first few introductory levels) has been wonderfully frantic.
Interface: 8.0 - During combat, the interface works fairly well. The default setup for the 0-9 keys is pretty solid. Repairing, attacking, and building can all be done on the fly with relative ease. Outside of combat though, there are a few issues I have with the interface.
First off, it is very hard to get any solid stat information out of the dungeon defenders tooltips. As a game that is very heavily based around stats & loot drops, this is a big issue. When assigning skill points, or deciding on loot that increases 'x' stat by 'y' number of points, you have no idea how much more effective you or your towers will be until you're in combat.
Also, certain bits of information, such as the number of defense points a particular tower costs, or a brief description of what the different towers do, is hidden behind a few layers of menus when it could be easily added as a tooltip when mousing over the appropriate 0-9 key.
They do have one non-combat UI innovation, however, that I expect to see popping up in games heavily based around loot drops. They have a "thumbs up/thumbs down" indicator that will allow you to tell, at a glance, whether an item is obviously better or worse than what you have equipped. They have a sideways thumb also, for when it's too close to tell. It was about 90% accurate - I occasionally disagreed with it when I was looking to max one stat in particular over another.., but nothing huge.
This indicator, combined with a "sell all" button that sold all of your non-locked items (you can select to lock items so you don't accidentally sell them), made getting rid of "vendor trash" incredibly easy.
OVERALL: 9.1 - The game has some issues that need ironing out (mostly bugs, disconnects, and a few interface quirks), but overall it delivers a very solid & frantic cooperative experience that I have enjoyed immensely. This title will definitely be my gaming circle's go-to title for the next little while.