I bought these for my Harley Benton telecaster, more for aesthetics than anything else, and I guess they come with the added benefit that the strings won't snag on any sharp edges.
They're not super cheap, but they're not really expensive either, and I'm considering getting a pair for my Affinity Strat as well, because they do look really good.
The only downside is that if you're installing these on a guitar with cheap string trees, you will most likely have to drill a small additional hole for the little knob in the front, which means you can't restore the old ones afterwards without leaving behind a couple of ugly holes.
Overall though, I do like how these look a whole lot more than the cheap ones a lot of guitars come with, and installing them wasn't a huge deal, although more work than just replacing a screw.
These are definitely worth the money for visual reason, if not for practical ones.