Asus were (and may still be) the world's largest motherboard manufacturer and have gone on to be in the top 5 PC vendors. They've historically been known for 'superior' (and thus expensive) product but since moving more into own brand PC sales they've spread their portfolio to the cheaper end of the market as well (without abandoning premium lines). I bought my daughter an X551CA for Christmas which is perfectly serviceable, though feels cheap (creaky), though no more than you'd expect for £270.
To cut a long story short, if I wanted a Windows machine and wanted to spend a lot of money (£500-£1k and beyond) then they would be top of my list probably, but at the cheaper end of the market (~£300) there are plenty of alternatives (eg Acer, Toshiba, HP) and it will be down to specs and looks, because they'll all feel pretty cheap.