This part of their terms and conditions, gives them the right to effectively suspend your site or bill you for overage whenever they feel you have used too much transfer:
Traffic is basically unlimited. However, traffic must be normal in nature and not disrupt that of other customers. If a
customer's traffic disrupts that of others, One.com reserves the right to close the customer's web space without notice
and/or make a separate charge for the traffic. One.com reserves the absolute right to decide whether or not traffic is
excessive. On closure of a web space, no refund will be made of any subscription paid in advance.
Hence its far easier to go with a company that quite simply offers you an amount rather than unlimited, where they decide when you have used too much (though you have to watch for companies offering absurd amounts of transfer - keep in mind 1Mbps of transit can cost from $4 - $24 dependent on the quality/tier/redundancy of the carriers in the mix and infrastructure so if someone is offering 3000MB of transit per month for a few quid, thats around 10Mbps of dedicated bandwidth, so that could be costing the company as high as £150 per month at the 95th).
There is room for manouvre thanks to the ability to oversell, which in theory and even in practice, does work to an extent, but if it looks far too good to be true...