Must admit first up that I'm no especially gifted painter but like a majority of hobbyists I can appreciate some good materials and tools to make my hobby experience successful. Painting for over 6 years or so has given me the opportunity to have spoken to lots of people who have given good advice and bad advice and here is the sum of my experiences.
There is three main range of paints that I currently use and after initially using GW range of paints I got given advice that P3 were the greatest paints (not really) ever so I should dump all my GW for this range and I wouldn't regret it (I did a bit). Nothing too severe against them but they too suffer some flaws in design as most things. Here is a summary of the ranges of paint I currently use.
P3 range: A little cheaper than GW's and also slightly more in the bottle. The colour range is acceptable but not expansive. Initially the coverage is brilliant and coats seem to glide on more. This is due to the formula which uses a liquid pigment which encourages coating adhesion. The liquid pigment is also the downfall of this range in terms of longevity and shelf life. The paint tends to thicken very fast whilst in the bottle (1-2years) and needs constant maintenance but almost impossible to revive if left too long. It also has a tendancy to develop blobs in the formula due to this quick deterioration. So summary of this range is that it's great when you first buy and use but expect to replace it in 2-3 years when the paint will be thick and blobby unless you regularly maintain. Disadvantages also are that the paint is not ideal for dry brushing and the metallics are just very hard to work with and dark with minimal tints.
Coat d'Arms range: The first range of GW paints and still being made by their manufacturer. A third cheaper or more than most ranges of paint and offers a solid if not a great range of colour options. Still very good for all kinds of painting but feels a little sloppy when used for dry brushing but nothing care won't fix. I tend to use this range for undercoats and glazes and the price will be a major draw card for hobbyists to this range. Metallics as with most GW's are the best I've encountered or though the range tends to have a tendancy to be a little too brightly tinted for my liking in the metallic golds and mithril.
GW recent range: I initially inherited a friends paint collection when I first started the hobby and I still have some of the paints he sold me at bargain basement prices. They must be in excess of 10 years old and many of them I have not really maintained regularly at all. The fantastic thing about them is that they are all easily revived with a bit of tap water (I know what you are suppose to use) and they are still going strong. The powder pigments used in the paint range makes it ideal for blending and dry brushing. Initially it seems that the coverage is not so good but its due to its design that you should layer on the paint to acheive different tones and shades off your undercoat and this took me a while to realise. Good solid work horse paint and has longevity and shelf life (currently over 10years). The metallics are superb and I've not found a range with better finish in metallics and its easy to apply.
GW's New range: Modern technology and from initial try it's a winner. They have fixed the coverage problems but still maintaining the same feel of a powder pigment paint. If this range proves to be as good a shelf life as the previous range then I will definitely invest. The new design to the use of their paints also speaks volumes to ever increasing innovation and I'm sold.