I've used a number of their kits in the past, and they were pretty good. The issue would be to make sure they're fresh. Since it's canned extract and crushed grain, you don't want a kit that's been on a store shelf for 6 months. Overall, though, I'd say they are a vast improvement over Mr. Beer.
I've ordered kits from Northern Brewer and Austinhomebrew.com that I thought were much better, though. They actually crush the grain and package up the ingredients when you order, so that it's the best quality you can buy. The extract is much fresher, since they sell tons of it. Both have websites that are easy to navigate. Austinhomebrew is a vendor active on this site, and he responds to any questions or concerns right away.
The nice thing about the Brewer's Best kits, and the other kits from the vendors I mentioned is that they come with excellent detailed instructions. Once you're really comfortable with the brewing process, you may not need that- but until you are, it's wonderful to have step-by-step detailed instructions!
I also like the austinhomebrew.com website because of the sheer variety of kits. They have clone kits of just about every commercial beer you can think of, as well as "style" kits like Brewer's Best has. I'd only caution you to stay away from lagers (since the temperature control is crucial, and so is more finicky yeast) and from beers that have things like "oaking" and other stuff. Otherwise, you can make just about any style you can think of!