We've just completed installation of two 80mb/s bonded internet lines to Marykirk.com, from two separate locations in Montrose.
What this means is that even if one location goes completely off line, the service still runs. We're using a service from Evolving Networks here in the UK, and despite BT's best efforts (4 line faults in a single week), its working well.
This means that we have 160mb of bandwidth via Evolving Networks and 80mb via Zen, and 60mb of upload capacity.
Bottom line. If your business needs good fast internet, but you can handle a (BT) line fault from time to time, use Zen. If you absolutely need peace of mind, go with Evolving Networks bonded service. Its considerably more expensive, but perfect for our requirements.