Mr. Beer dominates the entry-level homebrew market for good reason. At a $30 price point, you can get started on your first batch of beer and avoid the larger financial burden and scary looking tools the more serious kits come with.
I spent a solid week reading up on the Mr. Beer kit - one of the best resources is a thread over at HomeBrewTalk.com - and learned more about the pros and cons. In the end, I decided not to buy one.
Business profitability-wise, the Mr. Beer kit is ingenious: they're selling you a 2.5 gallon plastic barrel with a spigot and a lid, and if you spring for the "premium" kit, you're buying some plastic PET bottles (2L soda bottles) as well.
The ingredients to Mr. Beer kits are mysterious. It's sort of like the instant-soup of beer making. They even boast, "Easy to brew, just add water." Refill kits run $15 a pop, and produce 2.5 gallons or roughly 20 beers. That's $0.75 a beer, which is cheap assuming the product is high-quality ($4.50 a six pack). Supposedly the beers are ready to drink in 14 days, which in hindsight I find hard to believe.
You're married to their kits with this system, meaning you can't just go buy yeast, hops & malt extract from your local homebrew store (LHBS) and whip up a batch. Since it's all instant, you also don't learn anything about the beer making process. For me, that education has been one of the most rewarding parts of homebrewing.
In the end, I decided to dive right into the deep end and purchase the $100 starter kit. I figured, why start with a mediocre method that could make me lose interest, when for a little more money I could start brewing like the pros?