Yes and YES! We moved into a bigger detached house from a terraced and noticed a difference straight away. The house always felt cold and the boiler is ancient and we can't afford to replace it yet. We got a company, eaga, to do a survery and do the insulation. DH had already done the loft, so we paid £149 for the cavity wall insulation. We noticed a difference almost immediately. The house just feels warmer and stays warmer when the heating goes off. I don't need the heating on as much, so it has already saved us money.
Absolutely, just bear in mind it keeps the house warmer in winter and cooler in summer. Ours has made a massive difference to the warmth in winter but in the nicer weather it is frequently warmer outside than in and we have to put and extra layer on indoors - great in the middle of summer to come inside and cool down but in spring/autumn it can be really chilly indoors!