As the name implies, this is a souped-up iPhone 3G, rather than a new generation of Apple's handset. It's more powerful, has a better camera and the ability to shoot video, and supports faster HSDPA data speeds. It's now available in a choice of 16GB or 32GB models, but it's the speed boost that's the main selling point.
What we like
The 3GS is like the iPhone 3G, but faster. That sounds like a small upgrade - few people have complained that the older model is a slowcoach - but the benefits can most clearly be seen in the way your apps boot up faster and run more smoothly. Gamers in particular will relish the improvement even before developers release 3GS-optimised games for it.
The beefed-up camera is also a big plus. Its 3 megapixels may lag behind the top-spec handsets from Nokia, LG and Samsung, but there's an excellent auto focus, a macro shooting mode, and the ability to tap on the screen to manually tweak the focus.
The new video feature is equally impressive, both in terms of quality and ease of use. There's a simple slider interface to top'n'tail your videos and uploading to YouTube is built in.
Meanwhile, the inclusion of a compass may sound like a minor detail, but activate it in the Maps app, and not only can you see which direction you're facing in, but the map also rotates accordingly, which is really useful when following directions.
What we don't like
Apple says the battery life of the 3GS is better than the iPhone 3G, but particularly if you're a heavy apps user, you'll find it runs out more quickly than you'd like. Sure, you can turn off 3G, Wi-Fi and location features to conserve battery life, but that's effectively neutering the phone.
Calling quality - one of the bugbears of previous models - remains on the average side. It's not dreadful by any means, but it still lags behind its rivals.
Something that isn't Apple's direct responsibility is the cost of upgrading to the 3GS. If you bought an iPhone 3G when it launched last June, you'll have to buy out six months of your O2 contract before being allowed to upgrade to the 3GS, or wait until Christmas. At that point, you could be six months away from the true next-generation iPhone, posing a dilemma.
The iPhone 3GS is certainly a step on from the iPhone 3G. Everything is faster, the camera is significantly improved, and the video shooting is very well done. If you want an iPhone and aren't encumbered by a long existing O2 contract, it's the one to go for. However, if you have the iPhone 3G kitted out with the new iPhone 3.0 software, you may prefer to wait and see what 2010 brings.