When myself and the then-girlfriend-now-wife bought our first place together, we needed to get a LOT of things sorted with our house before we could move in (to say fixer-upper was an understatement); double-glazing being one of them. But unless we went with the lowest quote for everything, it wasn't going to be possible to do it all and that's why we ended up using SafeStyle. The maxim of "you get what you pay for" was never truer - they were by far and away the cheapest and the product was satisfactory, but the fitting wasn't exactly top notch and after-sales was practically non-existent.
If you want a bargain and aren't too fussed about it being 100% perfect, as long as you go in with your eyes wide open and know about their sales tactics (which we did), then go for it. Having said that, we have since (19 years later) gone and got most of our doors and windows replaced by a local firm and are far happier with the results, so bear that in mind as well!