It's worth noting that you can charge your keyboard or trackpad while using it at the same time. However, you can't charge anything if your computer is completely turned off (though if it's in Sleep Mode, it will charge). I also tried charging using a standard 5-volt USB wall charger that would charge your typical smartphone -- that worked, too.
This rechargeable battery solution is more expensive than picking up a set of standard rechargeable batteries and using those instead of alkalines. But I didn't get much battery life when I used NiMH rechargeables (only a few days) and found the Mobee to be a much better -- and more convenient -- rechargeable option.
The big question is how long will it take for you to recoup your investment. Based on a $60 sale price (the product can be had for slightly less online), Mobee says it'll take a year but it's probably closer to two given that you can get good deals on bulk battery packages online. The good news is Mobee's rechargeable battery is rated at 500 cycles (recharges), so it should last several years before having to be replaced.
It can be hard to sink $60 upfront on batteries for a product that costs about the same price (the Apple Wireless Keyboard goes for around $60, the Magic Trackpad costs $69). Eventually, however, your investment will pay off. And on the green side of things, you don't have to worry about chucking all those one-use alkaline batteries in the garbage and having them end up in some landfill. (True, some people dispose of batteries properly. But many don't.)
In the end, the Mobee Magic Bar may not seem like the best deal at first glance, but it's a well-designed product that makes a good companion to the Apple Wireless Keyboard or Magic Trackpad and will save you money down the road.