For those of you that keep asking, I used this a couple years ago.
What does it do?
The downloaded file is an alternate executable from "World Editor.exe". This executable instead loads the world editor with files from it's own embedded MPQ that unlock many features not found in the normal editor. As an example, a Trigger Action "Custom JASS Script" was in UMSWE like a year before Blizzard put that in themselves. It also adds several groups of triggers, including Multiboard (a variation of Leaderboard that is a lot cooler to play with). It also unlocks the additional units that are normally hidden from the editor (Marine, zergling, etc.)
Will it crash my game?
Only if you try to unload it's embedded files into the main directory or replace the original World Editor.exe or something stupid like that. Also, if you use one of the custom Triggers provided in UMSWE you will not be able to load the map in the normal World Editor anymore, most like you will receive an error upon loading the map and the Editor will close. Always remember to make backups of your files/maps before trying to use 3rd party utilities.
Does it work?
It used to work wonders, but I don't think it's been set up for the current patch (1.22x). If someone wants to take the time to compare the embedded files with the current patch files and add fix the differences that would be awesome (I'm too lazy to do it myself).