Windscreen replacement has been discussed quite a bit on here previously.
The quality of the end result depends on the ability of the windscreen fitter, the quality of the glass and being the correct profile, plus the availability of the retainers and seals. I had a cheapish aftermarket screen fitted 'cash in hand' on an Omega I was selling to get a 12 month MOT, but the fit of the screen wasn't good - if it was correct at one corner, then it was proud at the opposite, etc etc and no amount of correction / repositioning greatly improved the finished job. I hadn't heard of the glass manufacturer before which made me question it's quality. The mouldings were slightly proud at the top corners too which didn't look right.
The other screen I had fitted in an Omega was a genuine VX which at the time was quite reasonably priced, and I would in preference get another genuine one, but difficult now to find. The fit was perfect.
If I was ever to need a new screen again, it would have to be a good known make, genuine retainers and seals (now hard to get the complete set), and an invoiced job so I would have some comeback if the end result was not as good as it should be.