I was fan of super mario level, but I create a mario was my dream too.
The first mario editor is mario worker, very simple to use but very limited and the version that I use had a background thank to a bug.
(I remember that I make a "scenario", but It was in a old computer)
My big brother showed me lunard magic, I used it, but it was hard because I got no tutorial.
I discovered SMBX in 2010 with the 1.2.2 version by a search, I was surprised how this editor was not limited with the layers and events, of course in my first utilisation I didn't know how to set the player position. (I placed a npc Luigi, thought it was the player)
Of course I made some levels that I give up, but this editor is fun. :p
Well, I found some screenshot to my very first project, to see my improvement. :p
There is a demo which was updated in megaupload.