It's amazingly protective: I dropped my phone on the rough tiles of my kitchen floor and the case really suffered - you could see deep gouges where it scraped across the floor. These seem to have faded into the background now, though, and there isn't a mark to be seen on the phone. Thumbs up for that. It's got a nice, rubberized feel to it as well, with what looks like plenty of room for error if you accidentally leave it in a spillage of water or something (I've done that too and no harm done). The grills for the microphones and speaker are exceptionally well-positioned - I'm convinced my phone sounds better in the case, possibly because the sound is being directed upwards rather than outwards. I particularly like its handy on/off switch, so you can opt to run the iPhone's internal battery down before switching on the battery inside the case. Even more handily, the charging circuit bypasses the switch, so you can charge up your phone with the case switched off. It charges up using what seems to have become the defacto standard for phone chargers these days - the Nokia/Samsungh/Blackbury take on Micro USB.
However, having said all that, there are a few things which I'm not so happy about:
* It's huge. I mean, fat to the point of being ugly. If you're buying an iPhone 4 or 4S for its slimness, don't bother using one of these cases - you certainly won't feel the benefit of the decreased size. Neither will you feel the benefit of the decreased weight - it's super heavy. Not surprising given the 2000mA battery, I don't suppose, but still. Friends of mine have subsequently purchased thinner and lighter with the same extra capacity.
* You slip the phone in by removing the top of the case. This process is waaaaaay too easy. If you pick the phone up by the top of the case by accident, you're guaranteed to drop the bottom of the case, and subsequently your phone. Of course, you could just not pick the case up by its top, but I can't help but think it wouldn't have been too difficult for them to build a propper locking mechanism.
* The charging circuit is very fragile. Admittedly, I've been using the case non-stop since I got it back in March, and it's now November. But that's only seven or eight months and already the cable is loose in the socket. I have to bend the cable into place before it will make a circuit and charge up.
* It won't fully charge an iPhone 4S. I've been getting it to about 80%, which admittedly is fantastic - it's definitely enough for an emergency power boost until I can get to a power socket - but it's not a full charge. In fairness to the manufacturers, 4Ss weren't released when they made the case, and you could easily get a charge and a half out of an iPhone 4, so this is more of an observation rather than a criticism or complaint.
It's now got to the point where I can't reliably charge it, so I've had to abandon it unless I know I'm desperately going to need the extra juice. Even with all the gripes above, though, I'm quite upset to part with it. It was a cracking device while it lasted.