Spyder's $90 PowerShadow i4 Battery Case & Charge/Sync Dock for iPhone 4 and 4S offers an interesting take on the charging case.
The black polycarbonate case, which offers 2000 milliamp hours of power, adds 2.6 ounces to your iPhone's weight. It measures 5.01 inches tall, 2.4 inches wide, and 0.79 inches deep.
Like many battery cases, the i4 consists of two pieces: the main base, which contains the battery and a dock connector plug, and a small cap. This cases' cap feels disconcertingly flimsy; off the phone, it lacks any rigidity and seems more fragile than I'd prefer.
Inserting the iPhone into the i4 case is simple enough. The case uses cutouts that expose the buttons, rear camera, and headphone jack. I prefer button overlays to cutouts for cases as a general rule, because I find that pressing the volume buttons and using the Ring/Silent switch takes notably more effort when those buttons are only accessible through slim cutouts; that's certainly the case with the i4.
On the base of the case is a Micro-USB port; you can charge the case by plugging the included USB cable into your iPhone power adapter or computer. But the i4 includes a second charging option that sets it apart from its competitors: You can instead place your i4-clad iPhone into the included dock for cordless phone-style induction recharging. The dock, like the case itself, has a Micro-USB port; again, though, you'll need to provide your own power source or adapter for the USB cable. (Note that Spyder says you should not stick the i4 into the dock sans iPhone. You can charge the case alone via USB cable, but should only charge it with the dock when your iPhone is inside the i4.)
In practice, the dock is a very nice feature. It docks the iPhone stably, and it recharges the case (and the phone within it), just as you'd expect. Not needing to fumble for cables is always a plus.
The i4 isn't meant to afford your iPhone any drop protection. In fact, Spyder warns you in the manual not to drop the case. The case doesn't extend past the screen, so think of the case as providing extra nick and scratch defense for the sides and back of your iPhone--along with the significant power boost, of course.
On the power front, however, note that Spyder offers some unusual recommendations for how best to use the case--ones I haven't seen elsewhere, and I've reviewed oodles of battery cases. From the manual: "We recommend turning on the i4 only after your iPhone's battery has dropped to 20 percent of full charge or less."
Whether your phone's power reserves have dripped as low as Spyder recommends or not, you press and hold the power button on the rear of the i4 case for a couple seconds to turn it on. Four blue LEDs illuminate to indicate the case's charge level, and those lights stay on for as long as the case is powered on. I'd prefer that my iPhone not glow continually like that, but this isn't a major complaint. When charging, the LEDs blink until the case is fully charged.
Macworld's buying advice
Overall, the i4 is a fine case, and the included charging base adds value. It's not my favorite battery case; I'm still partial to the Mophie Juice Pack Plus, which costs about the same and offers a better feel plus a bit more protection, though lacks the innovative charging base option. But if you want backup battery power and appreciate the benefits of the charging base, the i4 is certainly a fine option.