Well, I have some different opinions to offer. I offer them not with the implication that any of what's already been said is wrong, but just in order to establish that there are different ways of looking at the subject.
My principal interest is ship models - both sailing vessels and twentieth-century warships. I started building them in 1956, when the only paints commonly available (at least to five-year-old kids like me) were Testor's glossy enamels. (Well, the hobby shops also sold something called "Dope," but the aspiring modeler quickly found out that it inflicted horrible things on plastic.) I've bought several airbrushes over the years, and they certainly have their place; some jobs almost require them, and I greatly admire the work that my airbrush-enthusiast friends are able to do with them. Personally, though, I still tend to gravitate toward hand brushing. I'm sure that's partly due to the fact that I've never spent the time necessary to develop a good airbrush technique. But I also have to admit that I simply get more enjoyment out of painting by hand - and enjoyment, as far as I'm concerned, is what the hobby is about.
I inflicted the smells of turpentine and enamel paints on the household all through my youth and adolescence. I guess it must have been in about 1975 or thereabouts that I discovered Poly-S water-soluble paints. (I believe the original Poly-S formula was, strictly speaking, latex rather than acrylic. Don't ask me just what the difference is.) I immediately noticed several things. The stuff covered well, produced minimal brushstrokes (if any), and was available in an excellent variety of colors. (I still think the old Poly-S "Sahara Sand" was a great color for the unpainted wood decks of modern warships.) I had some reservations about its consistency (some colors seemed to brush much more easily than others), and I hesitated to use it for some purposes. Then I noticed that, whereas I had been in the habit of working at the bench for three or four hours at a stretch, when I used Poly-S I'd enjoy myself thoroughly for periods of five and six hours - and quit reluctantly because it was three or four o'clock in the morning. I realized that, without having any idea that it had been happening, I'd been doping myself on the fumes from solvent-based paints and turpentine. That did it. Since then I've been a whole-hearted convert to water-based hobby paints.
The old Poly-S brand is gone now, replaced by Poly-Scale. That's my current favorite. I get along reasonably well with Testor's Acryl, but I find some of the individual colors in that line have a strange, syruppy consistency that just isn't as friendly as Poly-Scale's. I would also urge anybody interested to take a look at the ranges of "craft" acrylic paints sold in arts and crafts stores. They come in a huge variety of colors, seem to work just as well on styrene as the ones designed specifically for models do, and (drum roll, please) cost about a fifth as much per ounce. You won't find "Panzer Yellow," "Burlington Northern Green," or "Haze Gray" in the ranges of those companies - but you will find an almost limitless variety of browns, grays, beiges, and blues. And, in my experience at least, the "craft paints" are easier to find. The Michael's Arts and Crafts store in the local shopping center carries two or three brands of them, totalling several hundred colors; I've even found the stuff at Wal-Mart. If I want to buy a bottle of Poly-Scale or Testor's Acry, I have to drive 35 miles to the nearest "local" hobby shop.
Mg.mikael writes unfavorably about synthetic brushes; I have to say I've had the opposite experience. There are some lousy synthetic brushes out there, but I've bought many in art supply stores (for extremely reasonable prices) that I consider excellent for model building. I've also spent more money on red sable brushes - including the famous Windsor and Newton Number 7s - over the years. They're wonderful tools all right, but in terms of texture, paint-holding ability, durability, and consistency (i.e., stiffness vs. flexibility), I find the modern synthetic ones every bit as good. They do handle slightly differently, but with a little practice they're capable of producing excellent results - for a fraction of the price that the best sable-hair ones cost. (One tip: for painting large areas - wider than, say 1/8" - select a flat, chisel-point brush rather than a round one. Flat brushes, for me at least, are much better at eliminating brush strokes.)
Some modelers have raised questions about the durability of acrylics. Here are three ship models that I've painted with acrylics: http://www.hmsvictoryscalemodels.be/johntilleygallery.htm . (Each "album" contains several pictures; click on any of the three to open up the other shots of that model.) The Bounty was finished in 1979, the Hancock in 1984, and the Phantom in (I think) 2006. All the photos are recent. None of the paint has been touched up since the models were finished. I think it's holding up about as well as any sort of hobby paint can be expected to.
One strange feature of hobby paints and painting techniques is that they seem to behave differently for different people. (Some of my friends say they just "can't" get the kind of results I get with hand-brushing. And some of them get spectacular results that I can't match.) That different modelers swear by acrylics and others swear by enamels (or Testor's vs. Poly-Scale, or synthetic vs. natural-bristle brushes) doesn't mean any of those individuals is "right" or "wrong." (It's worth noting, incidentally, that Floquil, Testor's, Pactra, and Poly-Scale are all owned by the same people nowadays.) To my notion, one of the most rewarding aspects of the hobby is trying out different techniques and materials, and finding out which ones work best for you.