'm not ashamed to say it... i'm addicted to digital cameras. Now, as a photographer / designer, this may sound like a fairly innocent claim, but it can be rather costly - especially as my latest purchase was the $1200 Nikon D90 12.3 Megapixel SLR.
'SLR' (single lens reflex) cameras have interchangeable lenses and a larger image sensor than found in your common-or-garden compact. This results in a better image quality and a huge variety of customisable options. Of course, this also means that they are more expensive than their smaller and more user friendly siblings - but you generally get what you pay for.

The D90 represents Nikon's intermediate option, with the cheaper D60 being the entry level model, and the top-of-the-range D3 aimed at serious (and rich!) pro's only.
The D90 has been designed so that it is easy enough for a new digital SLR user to use, whilst having enough advanced options to please seasoned photographers.

In terms of its design, the D90 looks a lot like its predecessor the 'D80' which was released a couple of years back. Apart from its slightly cleaner lines, the first obvious change is the huge three inch LCD screen on the back, which makes viewing your photos a pleasure.
Internally, the D90 has a set of impressive features including 4.5 frames a second continuous shooting, and a dust reduction system which cleverly prevents dirt and other tiny bits of debris finding their way onto the image sensor when changing lenses.

However, the reason that this particular camera has been grabbing all the headlines recently, is due to its ability to shoot high-definition video. Now, this may not sound like much - all cameras have movie modes right?... well, actually, wrong - the Nikon D90 is the first SLR to have this feature. The camera can record at 1,280 x 720 pixels, at 24 frames-per-second with mono sound. The downside to this is that focussing during movies has to be done manually, as auto focus is disabled whilst recording. The addition of the movie mode is a nice bonus, but overall it is a feature which I probably will not use.
As I mentioned in the first paragraph, the D90 offers 12.3 megapixels of shooting power - but megapixels often get confused with image quality, and have little to do with a cameras overall performance.

So how good are the pictures which the D90 can take?
My first impressions of the D90's image quality were outstanding - The colours that this camera produces are incredibly accurate and there is a great detail in both the shadowed and highlighted areas of my images.

Overall, I found some shots to be a little soft - but this isn't a huge problem, as the sharpness levels can be adjusted as part of the in-camera settings. I found that changing the sharpness from the default value of '2' up to '5' addresses the problem, and will produce better shots in the long run.
In terms of the image grain, or 'noise' as it is referred to in digital camera terms, the D90 does a good job in most circumstances. Offering an ISO range of 200 - 3200, noise is only noticeably introduced at around ISO 1200. For those of you who are unaware of what 'ISO' means, basically it represents the sensitivity of a cameras sensor. Using a higher ISO number will allow the camera to take pictures in lower light, but the result will be more noise in the final picture.

White Balance is excellent, and a novice user can happily set the D90 to 'auto' and let the advanced image processor do the dirty work for you. If you want to tinker yourself however, adjusting the white-balance (or any other setting on the camera for that matter), is really easy, and can be done via the well designed menu system.
Nikon SLR cameras generally have a great auto-focus system, and it is the same with this model. There are a range of options including face detection and 3D tracking modes, and the D90 is quick to lock-on to subjects, which makes composing your images a breeze.

The build quality of the Nikon D90 is excellent. It feels more substantial than my previous camera (the D60), and there is a noticeable increase in weight. That said, it isn't too heavy to hold, and feels like it would happily survive the odd knock or low drop (although i'm not going to test that out!). Overall, it is comfortable to hold and as long as you have a fairly light lens attached, will not make your arms too tired.
Below is a summary of the Nikon D90's extensive features:

Maximum Resolution: 4,288 x 2,848.
Supported Lenses: DX AF, G & D AF Nikkor & AI-P Nikkor.
ISO Sensitivity: 200 - 3000.
Metering Options: CW, S, 3D Colour Matrix.
Exposure Modes: A, P, S,M.
Weight: 620g (body only).
Battery: Li-ion EN-EL3e rechargeable.
Dimensions: 132 x 103 x 77mm.
LCD: 3" 920k-dpt TFT LCD.
Storage Cards: SD, SDHC.
In conclusion, Nikon has designed the D90 for both beginner and advanced SLR users, and it is instantly obvious to me that they have succeeded. The menu systems are clean and easy to navigate, whilst there are a range of options hidden away which will allow endless tinkering for the pro users who want a greater level of customisation.

Most importantly, the D90 is a fun camera, and although it sounds expensive, it offers great value for money when you consider firstly the build quality, and secondly the stunning images that it is capable of producing. The movie mode is an added bonus, but will no doubt appeal to users who are interested in producing creative digital video as well as high quality stills.
I highly recommended the D90 as a stunning piece of photographic equipment, Nikon have yet again designed a formidable machine - did I mention that it has a 0.15 second start up time? - that's also impressive!

I've also posted this review on dooyoo.co.uk under the same name.