I had one of the electric ones from Bosch that cost around £100 (model 1500 or something like that) and it was absolutely useless, always getting blocked, requiring every last cm to be pushed through and being very very slow.
When I started looking for a new one recently I noticed that they have now brought out some with a new cutting technique. My original had a disk spinning perpendicular to the direction of the branch, trying to chop it like you would a carrot. The new type of electric ones now have a radial blade, with blade being similar to a router blade or planer blade if you know what these are like, and they are reportedly more effective.
However in the end I went for a make called Eliet (I don't know if you can get them in the UK) which is a petrol driven model and costs around £500. This works on the principle of splitting the wood, much as you would do with an axe, before finally chopping it in to small pieces.
All I can say is that it is brilliant, I am more than pleased with it and I can do more in 1 hour than I could do in 2 days with the old machine, it pulls the branches through so you don't need to stand over it and the chipping it gives are perfect for spreading over your flower beds
I guess the old addage of you get what you pay for applies__