I am also interested in buying a plastic welding iron. I gave it a try with a soldering iron and that works well enough that I'm prepared to buy the real gear. My polyethylene repair may not be as strong as original, but the original broke and the repair has held through multiple drops off-road.
The "trick" as far as I understand it is to prevent burning while also getting good penetration. Welding and brazing metal have helped me learn about plastic. I used plastic shaved off another piece -- my hypothesis being that it would be a similar material with similar melt point and a good chance of melding together.
Yes it's ugly. Yes a real plastic welding iron would probably make it look better. (It is not visible at all from the outside.)
The two things I'd look for are interchangeable tip shapes and adjustable temperature with a guide to setting the temperature for different types of plastic. I THINK that you can shave welding rod off the pieces you're using to make a project though that may not work for all types of plastic.