I've been using the Mophie Juice Pack, an external battery that slips onto the Apple iPhone, for the past week and recommend it highly. The battery is about half the weight of the phone itself and fits neatly and securely onto the iPhone to provide a full day's charge for almost any usage scenario.
The $99.95 Juice Pack's power management is what stands out for me. The battery drains first (you can track the charge by pressing a button on the back that shows a remaining power indicator), keeping a charge on the phone until the external battery is no longer needed. So, when you've exhausted the battery, just pull it off and put it in your bag. The iPhone remains charged and ready for another few hours heavy duty.
A couple downsides. While the power passes through, the Juice Pack doesn't seem to pass through synching data. I had to connect my iPhone separately to synch calendars and video. The other problem is the non-slip coating on the black exterior of the battery: it wears off. After just a couple days in my pocket, the corners and edges of the Juice Pack were worn looking. I suspect the anti-slip coating peels off completely.
These minor shortcomings aside, the Juice Pack a must-have for travelers and people who use their iPhone all day long. Last week, I flew across country listening to music and watching a complete movie along the way, arriving home with the Juice Pack still half-charged. By contrast, my iPhone would barely have made the trip without running out of power. The Juice Pack promises, and seems to deliver, 8 hours of additional talk time, audio playback of up to 24 hours and video for up to seven hours.
The second product note is a gotcha I haven't seen anywhere, including on the vendor's site. The Simple Tech Duo Pro Drive, a 1 TB external USB/eSATA drive, cannot be formatted by Mac OS 10.5.2, and the company can't tell me whether the fix will be in the upcoming 10.5.3. Apparently, no dual-drive enclosures work with the latest Mac OS Disk Utility app. The solution, if you have a Mac that still runs OS 10.4, is to format the Duo Pro drive using the Disk Utility that version of the Mac OS.
So, I've got a nice 1TB brick here until Apple fixes this.... Eccch.