Okay, most people with broadband routers will have configured some level of firewall protection on that device but for those with only ADSL modem connections, this could be a very useful service. The basic settings allow you to choose your level of protection from off, low or high and there's an advanced level for fine tuning the level of port blocking applied. The firewall service is off by default and you'll need to disconnect and reconnect before any changes are registered.
I've been a PlusNet subscriber for about 18 months now and (fingers crossed) it's always been a reasonably solid and reliable service.
NB: PlusNet already offer subscribers the ability to restrict access to binary newsgroups and P2P services so that's also a useful option for concerned parents, etc.