I have used them in the past and have nothing against them apart from being quite expensive and found them lacking enough solder, although you can add an extra bit of multicore solder before heating the joint.
My preference is a similar method but to use thin walled brass tubing of matching wire diameter.
Cut to a suitable length, the wires are soldered from each end and finished off by heat shrink sleeving.
I use this method when replacing motor cables where the space is restricted.
With correct selection of tubing you would not know there is a butt joint under the sleeving.
The brass tubing is freely available on Ebay or from model shops.
I also use this method when terminating wires direct onto motor coil wires - photo 1 shows the various stages viewed from left to right, photo 2 the finished job,
