The quality of Ribble stuff is fine. And you'll struggle to get better value. THey pile it high and sell it cheap. THe frames are the usual straight out of Taiwan/ China jobs that most people flog these days. The reports I've had of the recent bikes are good, although most say the handling is not as good as on bikes of similar spec which cost more (i.e. 1500 gets you an all carbon ultegra kitted out Ribble, but 2200 gets you a similarly specced bike with 'Scott' or 'Wilier' written on it). THe difference is notable if you are putting the bike through its paces (ie racing) but otherwise not worth worrying about. From that perspective RIbble are fine. The sizing can be a little odd and I don't know if it's been standardised no but some frames used to have really long top tubes and others were shorter making it rather difficult o get sizing right.
Customer service is dreadful. I know all the reasons why and won't go into them here. They exist because they are so much cheaper than everyone else, not because they offer a great service. We all know them from their online retail, but ask around in the trade and there are plenty of stories of sharp practice, in particular involving their one time frame builder who now builds frames in his own name. I don't buy from them for that reason, but it's a personal decision