Considering these as they are cheaper than recirculating ball slides (good ones anyway) and am struggling to get some chinese MGN12 rails that are any good.
The Igus linear rails I've seen are pretty nice, but they aren't completely zero-play - as you would find with a good recirculating ball slide. If you are using multiple slide blocks on 2 separate rails, though, you should be able to bias them slightly when mounting to take out any play.
