I do about 50/50. Sometimes the kit is just so beautiful with so many fabrics that it is a no brainer. I got one in CA and have never ever seen the fabrics out here in Indiana.
If the price is right, a kit can be a great buy. I have gotten some great deals at garage sales or on Ebay and enjoyed making the top. I have no problem inserting a different fabric if one is short or even changing it a lot, if I want so I dont worry about the kit being perfect ( if the price is right).
I like a lot of fabrics in a quilt and it is hard to get a lot of variety all at once in a quilt store becaue I dont have the time to stand around while quarter yards are cut of a lot of fabrics.