Over the past year I have tried 3 of them from Currys. One thing I would say is don't worry about making USB a priority against one which has a battery, at this stage I have yet to replace a battery, they just seem to go on forever.
Initially I tried the cheaper Advent one which they did ..around £22.00 .. not because of the price but because it was whisper quiet ... steer clear of that one. All of a sudden my typing ability decreased. There seemed to be missing letters in words and worst of all it often had a lag whereby a few seconds after you had finished typing the text might appear on the screen. This was duly returned and a replacement issued. The same problems continued and the replacement was returned.
The one I got in it's place is the best one one I have ever used although it is battery powered... Logitech K400 ... it is whisper quiet and has a scroll pad instead of a mouse.
This feature is a real plus and I don't think I will ever go back to using a mouse again after this. I would definitely recommend it.
Go along there and have a try of the keyboards on display. If a number pad is not something you need they have the Microsoft Arc on offer at less than half price at the moment.