We had a 8kW Mitsubishi split unit installed at the beginning of June last year and it was the best thing we've ever done. We have indoor units in three rooms (kitchen, main bedroom and Living room), but if you have the doors open it'll cool the whole house nicely when all three are running. Performance-wise they blow the mobile units away. We used to run two upstairs and there is no comparison. During that spell last summer it was 37C outside and inside was at a steady 21C in pretty much every room. I cant imagine what it would have been like without them as this house is a new build and holds heat in very effectively. Power usage is not too bad either, around 600-700w when running. Fortunately that is not an issue now we have solar, but at around 65p/hour at current prices, its worth it for the comfort.
I've yet to try it in heating mode, however now we've got the solar panels I might give it a try and save on some gas costs too.