Very well made. I have a full set of PolarStar parts on my BAR10 right now and its my absolute favorite gun shooting at 580fps and effective to almost 300ft. I've toyed with Laylax parts on a friends gun and this is considerably superior to their spring guide. The only thing to watch out for is that this will not work with the standard piston. And I've only tested it with my 45 degree PolarStar piston, so I can only confirm that as a suitable replacement to go with this.
just because it needs a new piston (hardly a con)
Makes the stock part look like the chunk of crappy plastic it is
Needs new piston or won't work
(but hey, you should buy one of those anyway)
Maybe a tiny bit pricey