In terms of overall performance the GH1 is, not too surprisingly, pretty much identical to the G1. The start-up time is quick, although from power-on to taking the first picture is a fraction over two seconds. That's pretty quick by compact camera standards, but there are plenty of DSLRs that are quicker. In single-shot mode it has an impressively fast shot-to-shot time of approximately 0.7 seconds, while in standard continuous shooting mode it can rattle along at around three frames a second, which is pretty quick.
The autofocus system is amazingly fast for a contrast detection system, and works exceptionally well in low light thanks to a bright AF assist lamp, although for some strange reason this had been set to "off" by default on my review camera.
Overall picture quality is, as you would expect, identical to the G1 as well. The new 14-140mm lens performs extremely well, producing very little optical distortion at any focal length, and showing outstanding corner sharpness with no trace of chromatic aberration. The level of recorded detail is very good, certainly on a par with any good mid-range APS-C DSLR.
The only problems with image quality are ones that are common to Four Thirds sensors. Dynamic range is certainly superior to any 12-megapixel compact, but it's simply not as good as most rival DSLRs. Also Four Thirds sensors have simply not proven to be as good at coping with high-ISO noise as most APS-C sensors, and the GH1 does suffer from significant image noise problems at higher ISO settings, at 800 and 1600 ISO. I've seen worse, but I've also seen a lot better from some recent rival APS-C DSLRs. Upon reflection I was possibly a bit lenient on the G1 in this department.
Overall the GH1's image quality is very nearly as good as that of a similarly-priced APS-C DSLR, and its video quality is nearly as good as that of a dedicated camcorder. The question is, will people pay £1,300 for a hybrid of the two?
There's no doubt at all that the Panasonic Lumix DMC-GH1 is a very nice camera. Build quality is excellent, and the camera body and control layout are very well designed. Performance is first rate, image quality in both still and video modes is certainly acceptable, and the extensive list of features and options give it a lot of creative potential. The only question is whether you want to pay such a high price for it.