Nikon D90 - Panasonic GH1, Panasonic FZ28 - Panasonic GF3
As can be seen quite a size difference! The Nikon D90 is shown with a 18-200mm 10X zoom lens and the Panasonic GH1, next to it, also has a 10X zoom fitted. By comparison the FZ28 bridge camera has an 18X lens built in (the latest have 24X zooms). The GF3 is shown with the very compact X PZ 14-42mm 3X zoom lens. Even the smallest Panasonic GF3 is much bigger than a standard pocket compact.
Canon IXUS 80IS against a Panasonic GF3X
Along with the size of the cameras is the weight and the images below show the differences between them.
As can be seen quite a deference in weight! All had batteries and SD cards inside.
If you have to have the D90 on your shoulder all day you really know about it! All the others can be carried all day without really noticing they are there. Even the GH1 when fitted with the 10X zoom lens only weighs 709g, almost half the weight of the D90 and its similar 10X zoom lens! When you carry them normally you will have a suitable protective case so the image below shows the difference in size. The GH1 will fit in the FZ28 (Apex 100 AW) case when fitted with either the 20mm pancake or X PZ 14-42mm lens making a great travel companion.
GF3 (Apex 60 AX case)
GH1 (Rezo TLZ 10 case)
I would say that most people will find the compromise of price, performance and convenience of a bridge camera, such as the Panasonic FZ150 or similar, will serve them well. For those who want better image quality and good video performance I suggest looking at the Panasonic G3, but if video is important then the GH2 (replacement for GH1) with the 14-140mm lens is a good choice. This could be complemented with a fast lens such as the 20mm f1.7 pancake or the equally compact dimensions of the X PZ 14-42mm lens. At the end of the day if you are really obsessive about image quality and control then a full DSLR like the Nikon D5100 or new D3200 is a better solution as long as you accept the larger size and weight penalty. These DSLR should be paired with a 10X zoom for general use (AF-S DX VR 18-200mm 3.5-5.6G ED II) and a prime lens with large aperture for good low light shooting (AF-S 35mm f/1.8 G DX).
Complementing a GH1, GH2, G3 or larger DSLR could be a camera such as the GF3 (now GF5) or possibly GX1 for times when a smaller quality camera is desired. Of course the Olympus PEN range is also a possibility for this role as it the Sony NEX 3, but the NEX range suffers from larger lenses so can negate any benefit gained by the compact body.
One tip for those who do go for the GH1,GH2, GH3 or in fact any micro four-thirds or DSLR camera with a flash hot shoe is to obtain a good external flash gun. This can really improve your internal shots as it stops the need for high ISO values, which introduce noise in the images, and provides a nice even illumination, especially when bounced off a ceiling.
GH1 fitted with a Metz 48 AF-1
Top view of Nikon D90 - Panasonic GH1, FZ28 Bridge and GF3X
With the 14-140mm lens on the GF3 it becomes a bit of a handful!
In many cases when you compare the GH1 vs the GF3 for many the GH1 (or GH2) is a better choice if you only want one camera. When the GH1 or GH2 is fitted with the new X PZ 14-42mm lens it makes a compact and relatively light package to carry all day. Only the new Olympus OM-D E-M5 can really compete in the size and functions stake but at a cost of almost double (for the body only comparison).
Panasonic has just released the GX7 which I would say, when paired with the X PZ 14-42mm lens, now makes the ideal only camera you would need. Like the Sony NEX 6 it has a small electronic viewfinder t complement the rear LCD touch screen. Unlike the Sony though the viewfinder can be tilted at an angle to get it into an optimum position. Also the GX7, unlike some compact system cameras, has a built in flash (as well as a hot shoe). For a camera with the performance of a G6 but the size of a GF3 this is a real feat of engineering!