This is a great thing to have! Please, keep moving with this amazing tool! Here is my first impression:
1) It is much better to have in-game rendering, rather than its simulation with map editor. Working with this I will know for sure that my map would look like the thing I'm currently working on.
2) Having an in-game resolution really helps! It is much easier to detect small mistakes on the map when looking from a game perspective, especially when working on a big map.
3) It runs way more smooth. Currently, I'm working with a huge mapblock (50x70x5) and map editors lags kills me. In your editor it's perfect. Tomorrow I will proceed with your editor, it's way faster and definitely much enjoyable!
4) In-game feel - when working in the map editor I am "working". I love modding and level design in particular, but it still. I was not working for a log time, just tried a few things. But all that feeling that is created by using vanilla-style UI and game rendering makes me feel more like I'm DMing a D&D session and I am in process of set up a dungeon on the table from tiles, and it is a part of a game we are playing with my friends. Hard to explain, but I like your environment much better than working in window forms.
5) Browsing mod's maps. It still not perfect, you know, there could be some more additions to that within terms of UX, but it's already better - you just choose a mod and the map, instead of those additional manipulations with editor settings I always hated.
6) God bless you, Undo button!!!11
It is a great progress for an alpha, I'm sure you have some things in your backlog and I really hope you will continue this amazing project!
UPD:
I can confirm the first impression. Tho mapmaking is not the most critical part of my current project I'm working on, it's still important. So I have to add - handy UI and much faster placing of new tiles in the general workflow for me compensate lack of filling and cut/copy/paste functions. It's obvious, that they would be very welcome, but now this editor is my main mapping tool and after a few days of working there I don't want to return back to MapView 2. All my experience is recorded (tho, it's all in Russian, sorry). The process is much more enjoyable, I guys have to try it, it's like some magic opens when you get the flow of it because in-game rendering feels like playing, especially when the shadow of the upper floor is calculated!
Pls, keep moving with this awesome project!