I used the plug as a ZigBee repeater 90% of the year and to turn the Christmas tree on/off using SmartLighting the other 10% so if you need to extend your ZigBee network or plan on adding ZigBee sensors that might be a little far from the hub it might be worth having around.
Here in the states the hub is on sale for $50 (~ £38) and the starter kit is $150 ( ~ £114). If ours was $100 for the starter kit I would have bought a second for the plug and motion alone and sold off the rest.
I would say it is worth the £99 package because Smartthings really is not that plug and play apart from the very basics. What you get in the kit then is some basic “things” that on the whole reliably connect, reliably work and that do pretty much plug and play. So, it is a good way to start understanding the Smartthings concept and thinking about what you might want to do with it. After that, you can start downloading privately written programme codes through the interface and adding non official “things”, which expands the possibilities enormously but at the beginning I found that it does require quite a few hours and alot of head scratching if you are non IT based !
So, this eases you in…if you struggle with these, I think you won’t get very far with the more advances stuff !!