I have used Glasses Direct, and found them good, but they are not anywhere near as cheap as the headline prices suggest.
I wear varifocals, which obviously increases the cost of the lenses. On your first varifocal order they ask you to hold up a credit card (a loyalty card is fine) so they can measure your pupil distance, and that measurement is kept on file afterwards, so you only need to do it once. They have 2 for 1 offers like the High Street stores, and often have money off deals. Overall, they are a bit cheaper than somewhere like Specsavers, but not massively so.
What I like about them is that there is a good range of frames, and as they aren't on the high street you are less likely to see others wearing the same ones. You can also wear the glasses for 30 days (90 for varifocals) to see if you like them, and if they aren't working for you you can return them.