They work great, but as alluded to above the handover between base stations is generally not an automatic thing, meaning you have to select the extender manually if you want to use it instead of your current wifi network.
You can buy more expensive units that will have a seamless changeover, but considering the price difference I reckon the added step of selecting the extender on your wifi list is worth the hassle. If you get the positioning of the extender right (as close to the centre of the house as possible) you might find that you are able to connect to the extender from all locations anyway which negates the need to keep changing between networks anyway.