I've bought kits early on when I first started out. It can be a great way to end up with a quilt that you like as designing one on your own sometimes results in a surprise which can be good or bad. You might run out of fabric if you're doing the planning yourself and often times colors and designs can be less than you hoped they would be "in your head."
If you find one you love, check the price and see if it is worthwhile to you. If you don't have a stash yet or you aren't near quilt shops or fabric stores that carry great fabric, a kit can be a good buy to save you time and trouble searching out the fabric you want. Now that I have a pretty good stash and several UFOs, I seldom buy a kit. It can be great for a beginner though, in my opinion.
Brenda