I do it because I enjoy it. I've built about a third of the furniture we have, and have gotten almost everything else, except for upholstered pieces, used from second hand/antique shops and auctions.

I bought a few kits when I first started out and they really helped me get an understanding of joinery and how furniture should be built. They also enabled me to complete some good looking pieces in a reasonable amount of time without making a lot of firewood.

I have to second what a lot of others have said, unless your time has no value to you, you won't save any money especially compared to buying good quality pieces used. But it can be alot of fun, especially when the weather is cold