Everything is done that little bit better. The iPhone slots snugly into the case. The top third comes off and the iPhone slides in with millimetre-precision. The inside of the case has a dock connector and the top of the case slides on top of the iPhone and locks with a reassuring click. It isn't going to come apart without your help.
Around the edges of the case you'll find replacement power and volume buttons (since the existing controls are covered), and gaps for the mute switch and headphone jack. In addition you'll find slits just below the home button, these redirect the external speakers of the iPhone so sound is pushed upwards. The effect is subtle, but it does provide slightly more volume and casts audio a little wider. On the bottom right corner you'll find an on/off switch which toggles the case charging on and off and a useful battery indicator button on the bottom edge. When pressed this illuminates up to four status lights to indicate the remaining charge in the case. A simple, but invaluable feature.
On the bottom left corner you'll see a micro USB port. This charges the Juice Pack Plus case and the iPhone, but the clever bit is it charges the iPhone fully first before the case. Consequently there is no downside to charging the phone this way as opposed to using a regular dock. Your iPhone can now be charged with any standard micro USB cable too and it charges at the same rate as with a dock connector. Since the port can handle power and data the iPhone can happily sync with iTunes through the case.
What's more the Plus lives up to its grand battery proclamations. You'll get at least two days of battery life on a single charge even with heavy use. This means over the course of 48 hours 4-5 hours of music listening, approximately two hours of video and web surfing, Spotify use and two hours of satellite navigation (we used the TomTom app). With light use the iPhone will make it through three days and if you switch off battery suckers like 3G, WiFi, Bluetooth and GPS it will see you through a working week. In short the Plus will make your iPhone last like your old dumbphone.
The problem is this doesn't change the price. £89.95 is still a lot of money for a case. Especially since Apple changes the shape of the iPhone with such regularity meaning it isn't likely to fit the iPhone 5. With the next iPhone due in June/July you'll need to be locked into a long term contract or be a frequent long distance traveller to really justify the cost. Especially with Mophie selling the Juice Pack Air, a 1,500mAh variant (also 70g, but 1mm thinner) for £60.
It is hard to knock a product which achieves everything it sets out to do. The Mophie Juice Pack Plus is well designed, durable, full of clever touches and delivers on its lofty claims of doubling iPhone battery life. That said we can't help but think almost £90 is a lot for an iPhone 4 case, especially when it is for a phone three quarters of the way through its life cycle. It's a cautious recommendation, but value for money will depend entirely on individual circumstance.