I was wondering this myself. It's surprising that I don't see any user maps around... I was hoping to try some out.
I wonder if this is because of the rather lackluster map editor.
I tried messing a bit around with it yesterday and I found myself getting very annoyed very quickly. Its not impossible to use by any means, but there are some really baffling design decisions.
For example, I spent the first 10 min cursing because I could not figure out how to paint tiles with landscapes/textures, but then after a while I discovered that first you must left click a tile and then right click it to paint that tile. Why can't I just pick a texture and then left click on a tile like in almost every other editor ever?
Also, I can't find any options that allow you to choose the size of the area you want to paint. This means that you have to painfully paint only 1 single tile at a time, so it takes forever to create large landmasses. If it is possible to paint a larger area at the same time then the option to do so is well hidden.
Also, it appears that it isn't possible (again, if it is then the editor does nothing whatsoever to make it obvious how) to actually undo any changes made. Pressing Ctrl+Z doesn't undo anything, and I don't think other functions like copy or paste work either.
Another example of editor stupitidy is when I wanted to select which tiles to support cities. I had already made some grass areas but could not figure it out how to make them support cities. After like 15 min I gave up and went to do something else. Then later I decided to go back to the editor and then once I loaded the map the grass tile areas for some reason all supported cities now and had the typical food/production/mana yields on them. I am not sure how, but I think it is possible that the tiles I used just support cities by default while others don't, and that for some reason the actual tile yields just did not show up before I re-loaded the map.
Speaking of the map now showing things you add to it, sometimes when you zoom out and the map changes to the zoom out map mode some of the things you just added to the map don't appear on it. A bug?
I am going to be honest here, while perhaps not terrible the map editor kind of sucks.
Its not so bad that it can't be used to create any new maps. I am sure that its completely possible to create good maps with it. The problem is that its just so painful to use that I can easily imagine that its one of the key reasons for why we don't see people posting a lot of user maps.
I now its probably not a big priority, but StarDock should really update the editor to make it less crap.