This probably isn't all that helpful, but when I had cable broadband from NTL at university in Nottingham back in 2003 it was massively unreliable. It would go down on average every week. When it goes down, it could be for an hour, or it could be 24 hours.

When it did work, it did have fantastic throughput though.

For a backup connection, it is probably ok. Just don't rely on it.