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I've repaired a few that seem ok but the little wheels have broken etc. They have been the type with heavy duty steel walls that replace the studs. Not sure I would want to retrofit one but if you're in the process of building a stud wall they can work.
The wheels usually have some adjustment. Mostly difficult to reach, needs some dismantling of architrave and you need to get the front runners right out of the frame to reach the back ones sometimes. If you adjust them properly they will be smooth. Having a solid fire door blank type of door seems to help them run smoothly too as you tend not to lift them when you pull them.
I'd say they're only really a good choice when you are not going to use them much.
Pocket sliding doors - are the premade ones all shit and should I go for an externally hung one? I don't think I've ever used one where the action was nice and they always felt a bit wobbly so my mind feels that putting all the mechanics on the outside where I can access it might be a good idea.
Something like this https://www.directdoors.com/products/ermetika-evokit-single-door-system-cassette
vs something like this https://www.fhbrundle.co.uk/products/41002__Sliding_Door_Kit_for_Solid_Door_Satin_Polish_304_Stainless_Steel