For me, the FD website was pretty weak. Yes, it did most of the things that i wanted it to, but to me the interface felt clunky and poorly thought out. i left them a while back now so i'm struggling to remember all the things that bothered me and there wan't much but it certainly wasn't a great interface.
nagging issues included:
-Always trying to open in a new window as opposed to just in a new tab
-poor website scaling so that when i forced it to open in a new tab it looked cramped and dark (ok - i accept that this is slightly my own fault!)
-When you log in (or pay, i cannot remember now), you have to click through a screen to select your account, even when you only have one account with them. so you have to select one option from a list of one, waiting for the page to load each time. tedious.
-the PIN-Sentry thing was a pain.. and you only had limited functionality without it.
That, combined with some good switch offers from competitors made me swap accounts (which was very simple and easy to do)
That said, you are correct, i don't recall any connectivity problems. But although their phone service is second to none, i would much rather get things done online as phoning anyone (even the best people) always felt like effort i couldn't be bothered with.