My experience....
Customer service is pretty bad, I have come across a couple of good eggs but they are the exception rather than the norm.
The supplied router is poor, I was suffering from the FEC/CRC errors on uploads which seem fairly common. Connection used to drop at least once in 24 hours (usually in the early hours of the morning). Changed to a Billion router (as they finally started giving out the required usernames and passwords) and these issues have disappeared.
As for speeds, I am on the 40/10 package and sync at 40000/7138. I suffer from significant drops in single threaded download speeds on a daily basis during peak times. I expect this is due to insufficient backhaul capacity. It's been getting worse over the last few months.
This starts between 6-7pm and goes on until between 10 and midnight. Here are some speedtests (all taken via a LAN connection) from this week to illustrate the issue:
Monday
https://www.thinkbroadband.com/speedtest/15196695876...
https://www.thinkbroadband.com/speedtest/15196758782...
Tuesday
https://www.thinkbroadband.com/speedtest/15197571298...
https://www.thinkbroadband.com/speedtest/15197660847...
Wednesday
https://www.thinkbroadband.com/speedtest/15198367218...
https://www.thinkbroadband.com/speedtest/15198442718...
Thursday
https://www.thinkbroadband.com/speedtest/15199236435...
https://www.thinkbroadband.com/speedtest/15199363757...
Friday (tonight) started dropping from around 3pm
https://www.thinkbroadband.com/speedtest/15199363757...
https://www.thinkbroadband.com/speedtest/15200211688...
As you can see, my multi-threaded download speed is hardly affected if at all. It's just the single threaded performance which drops dramatically. Most frustratingly for me is the affect this has on the performance of my works VPN in the evening.
This performance is significantly worse than I received from TalkTalk retail a couple of years ago. TalkTalk would have moments of significant slowness but it seemed to be only for short (non-regular) periods and seemed to affect both multi-threaded and single threaded downloads.
My advice would be to avoid Vodafone broadband at the moment until we see if they increase capacity or keep running the service with such significant under capacity. With all their investment in advertising, PR and really low prices I expect things to get worse before they get better.
My contract is up at the end of May and I fully intend to leave Vodafone and go elsewhere!