It depends what you want to do, I was in a remote area one summer for 3 months on a line synced at 8Mbps down at approx 400kbps upload with BT.
We could do 2 Netflix streams at once without an issue, whilst someone else was doing general web browsing, the Sky movie downloads were available to view in around 30 seconds on the Sky TV on HD setting, Youtube played fine, as did Amazon Instant Video which played in 1080P HD if we had no one else streaming, or just standard HD if someone else had Netflix going... We often FaceTimed with friends and other family, granted the picture was pixelated for the other person viewing us (due to our poor upload) but we could see the other party in good HD quality... The only slowdowns noticeable to me were on torrents or large PS4 downloads, but I just set the console to not go into power save and to download updates in the background + stay on WiFi on standby for updates, so when I turned it on I usually had the updates.
There were 4 of us using the internet. Nobody else really noticed any difference to the connection vs our home connection, but the rest were not tech savvy. Certainly had no complaints.
Not saying it's ideal, but 8Mbps is just enough for most activities to work ok, it's when you get down to 2/3Mbps that things start falling over. That being said, I know plenty of people in Canary Wharf London who have syncs lower than 3Mbps and we often nip over play Youtube videos on the TV all night and order pizza etc, seems to work alright.