I've been building a guitar out of a cheap DIY kit that I had laying around. This kit did have string guides included, but they were the old kind which often bend the strings and don't do anything to reduce friction.
These roller guides do however minimize the amount of friction and bring the right amount of tension on the strings to get the most sustain out of them. You can clearly hear the difference in tone when they are installed.
What I personally don't really like about these string guides is that they have an extra peg in the front. This means drilling an extra hole, and I'd rather don't drill extra holes in a headstock (mainly because I'm not that experienced in guitar maintenance yet).
With all that said, if I need to install / replace string guides, I wouldn't hesitate buying these!