Just to give an update on this.
Tonight (only a matter of 10 minutes ago or thereabouts) I finished MDK2. It is longer than MDK1, and I did complete the last level with all three different characters to see all the endings.
However, MDK2 did not turn out to be what I was hoping for. MDK1 was great, much fun, little bit of silliness here and there but not too much, was very fast run and gun action.
MDK2, on the other hand, well, it just seems that they tried to add to the game with new characters (and therefore types of level) and a lot more humour. But instead of adding to the game I feel it took away from it instead, when compared to MDK1. For starters, I liked the atmospheric drop and the end of level energy streaming parts. Those were gone. The first level does have an atmospheric freefall drop, but if you get hit just once it restarts. Then the minecrawler itself didn't feel right.
In the first game you felt you were in a big mechanical city/landscape, which fit with how the minecrawlers are described in the manual. But this one (there is only one in the game, too, for the first level) was just too weird. For instance, you rise up through a tube and come out in some floating area. However, look down over the edge and you see the floor of the earth with the cityscape around it and NOT the minecrawler. It just didn't make sense in that way. Plus the maps were broken into parts so if you fall off those floating bits you restart from the previous save point rather than land somewhere (Kurt has a coil parachute after all).
As for playing the other characters, well, they give a very different gameplay experience. Max is just all out shoot everything but with some annoying limited fuel jetpack moments. And Dr Hawkins was initially like an action puzzler trying to figure out how to use the items but his later levels were less so.
Don't get me wrong, the game is still fun and I can't complain for the hours of fun I had playing it at just $2.99, so it was a good decision to get it. It just wasn't quite what I was expecting from the game considering how much I liked MDK1 and so there was a little bit of disappointment. I can't see how MDK2 HD would be worth it though, so if there is no free upgrade from GOG (like how we had the Stronghold HD update) then I don't think I'll bother with it.