I'll add my two penny-worth here, since I migrated to Vodafone just over two weeks ago.
Generally I have not noticed much problem with download speeds, although at peak times there can be a slight slowdown.
However, we don't have big download requirements, and if the cost of Vodafone FTTC had not been so cheap we would probably have stayed with ADSL. We also have a VERY good line, with a maximum of 190m of underground copper between us and the fibre cabinet, 70m of which is the single line on our property and thus without the possibility of crosstalk. We are also in a village location, so unlikely to experience as much local congestion as many others.
As always, YMMV.