What Exactly Is...
 

So exactly what are the joint gattai series? Is it like, Legos but in action figure like form? I only ask because I've been seeing them pop up on Mandrake for relatively cheap lately, and thought about buying one.

Joint Gattai is much like Mugenbine, having universal plug in joints all across the board so you can separate and recombine them in any configuration you want.

Here's TJ's review of Joint Gattai Magiking.
http://www.youtube.com/watch?v=YF1sgvUAR0Q

It's kinda of like the zordbuilder system in the US. Just on a much smaller even more simplified scale. But they are pretty decent min versions of DX robos. Though I believe the line was cancelled after poor sales :(

It's similar to Zord Builder, but it places a lot more emphasis on mixing and matching, forgoing actual transformation in favor of it, for better or worse.
It's a shame it was cancelled, but I don't think it was going anywhere, since a lot of combinations beyond the primary one wouldn't be possible (I think G12 is impossible to be formed, even if they actually made the 7 remaining engines).