Has anyone used these type connectors?

I think what you will find is that unless you are using "high temp" wire the insulation will melt before you get the solder to melt (with a good heat gun).
Now, that may be obvious from the discussion so far.
These type of things are best suited for Teflon wire in my experience.
(Look up the temp ratings)
They are very common in avionics work for that reason.
For my hobby work, usually take the sleeve off of a standard crimp and just use the inner barrel.
Then I add solder and heat shrink it.
I know that sounds like a lot of work, but other than the "environmental splice" that hallifaxhops shows, it is the best option for me.