I played MK from December of 2006 to about March 2007 (I think...) and got a good tasting of what it's all about. Here's the condensation of that tasting... yeah that's how I'm going to phrase that.
First the good, then the bad.
The first thing I think that set in for me was just how huge the universe is that you have to play in. For my BP friends out there, every solar system in MK probably has the same amount of total land as one planet in BP, the universe consists of a 135x110 grid, most grid blocks of this universe have about 3-6 solar systems in them! That's enormous and one of the high points of MK.
Another high point is the complex resource system. There are, aw crap I forget, but probably about 15-20 resources scattered around the planets, with about 4 rare resources, two of which are only found on maybe 0.01% of the planets. This makes for specific points of control and sparks war. you can build and fight with only a few of the rare resources, but like always it's better to have all of em.
Now for the bad. The thing that pissed me off the most about this game is the bugs in it... and there are a lot. Most of them are like loopholes in the rules, for instance it addresses the "if a tree falls in a forest and no one's around to hear it, does it make a sound?" issue, buy saying, "THE TREE DIDN'T FALL AT ALL!" Guns don't fire while no one's watching, meaning a player can move an entire fleet right across your defenses just by looking away... There are a bunch of other little bugs like this that take an awesome concept and just make it cheap...
Another issue is the game's lack of a support crew. While I was there, support was minimalistic, much like BP, but shortly after I left, the owner basically said, I'm out of money, and I'm not doing anything else. The player base is likewise small, but the thing is, it takes about three months to complete the research needed to compete in this game. With that long of a wait time, new players are going to be few and far between. Now, I'm for in depth research, but only when there are multiple branches, in a game that doesn't really reset, three months to be on par with a rather limited span of units is just ridiculous.
Lastly, the graphics are horrible... It attempts a 3d environment with 1990's graphic capabilities. The unit models need serious work, and the interface could use an overhaul. What is nice is the in game mail system, though chat is localized and in a huge universe that is a mistake!
There it is in a gigantic nutshell. My recommendation is to play BP instead of MK. BP may be somewhat smaller scale, but MK is just too slow and buggy to be fun.