Has anybody used the cheap Chinese MGN15 rails you can get on eBay?

Iv never built a CNC router before, and i was wondering if they are appropriate for a small desktop machine.
I hope to be able to cut aluminium. Machine cutting size would be approx 300x400mm.

I got a couple of 300mm long MGN15 rails last year originally intended for a 3D printer, but I decided against using them in the end. They have no free play, by they do stick a little in places. For a small belt driven 3D printer this would be a big issue, but would it be an issue for a nema 23 ball screw or rack and pinion driven CNC?

The other alternative I have been considering are SBR20 rails. Would these be a better option on a small budget (£500 ish for the mechanical componants).