I really enjoy the whole process of kitting up a project - sourcing the materials, ticking them off when they arrive and doing all the preparation before putting that first stitch in.
Over the past few years I've amassed a really good list of online suppliers both UK and USA that means I get almost anything called for. I update it regularly when I stumble across new suppliers - especially if someone starts stocking something in the UK that I could only previously get from the States.
I occaisionaly buy kits mainly when there isn't a chart only option. Even then I will usually have to change the fabric as I never stitch on Aida.
The one exception (and I know I'm not alone in this) is that I buy full kits from European Cross Stitch for Chatelaine Designs. There are several reasons for this - Martina's designs use many different threads, silks, crystals, beads etc and it can take ages to get everything from all the different suppliers. You can often arrange to pay for your kit in installments and as Chatelaine projects aren't cheap this is a wonderful opportunity. Also they come so beautifully packaged that it's like getting a birthday present from a special friend.
So that' s my answer - a bit of everything really.