I think Tamiya is a splendid paint in many ways. But it does seem that some folk don't read the labels. I don't know what the exact brew is that suspends the Tamiya pigments but it's a lot more than water. Because it will clean up with water means only that - it cleans up with water. After a long thread a year back or so (this might have been on ProModeller/Flory Models) most folks agreed with armor guru Adam Wilder in an article he wrote that Tamiya works better with Tamiya lacquer thinner. He is right - no question. A20 will do the job for standard airbrushing, but if your want to avoid clogging at low psi/light coat painting lacquer helps it lay down far better. As I noted earlier in this thread, if you don't believe me smell the stuff. It's simply different paint from a water based acrylic like Vallejo or (sadly not in USA) Revell Aqua Color. As also noted, if you're willing to walk the extra mile quality artist acrylics like Golden Fluid are terrific paints for both brushing and airbrushing when mixed with the right mediums.
I don't know why, but I've become a paint collector. $2.50 seems likes peanuts at any time, so every order means I pick up a few. So I've got them all. (BTW: I heartily agree that PollyScale Railroad paints are the best US acrylics and only lack the neat Euro bottles to put them up with the best period.) I think MM acrylics are good paint. But I've only used them in conjunction with acrylic mediums. I heard that they were prone to come off with masking tape. So on the underside of a FW I painted MM Lichtblau (lovely color) but added a little GAC 200 which is a hardener available at any art store along with a blizzard of acrylics mediums that will allow you to customize your paints greatly. The stuff works great.
That said, there's good reason beyond simple curiosity to like mastering water based acrylics. All of those solvents that hold Tamiya and Gunze together evaporate while you use them and join the air you breath. (Not that water based are "safe" because they have fine pigments in the air, so I suppose a mask is always a good idea.) And yes Tamiya has fine quality control, but I'd bet that bad bottles come from bad storage, not bad production.
Tack on another reason. Companies like MIG and AK are making a mint selling filters, washes etc that could easily be largely duplicated if one understood the mediums they employ. (Abteilung oils per volume are as expensive as the best grade artist oils: but they come in model colors and probably are designed for workability and fast drying over deep color. Hats of to MIG, but it wouldn't take much to make your own brew once you figured out what was going on.)
I wouldn't consider lacquers or enamels for painting, no matter how good. (I do rather like enamels for weathering: think AK Interactive may be right in arguing that you can do anything with them that folk do with oils without the long drying times.) But if you're going to us a water based acrylic, do not thin it with water more than maybe 20%. (I'd put some stuff called "Flow Aid" and "Retarder" into distilled water: it does help for smoother coats and slows drying a little.) If you want to use a water based acrylic very thin and spray at low PSI use "Airbrush Medium" available at art stores. (One bottle will last a very long time. All mediums are cheap per volume.) It's a kind of very thin polymer that will not break down the molecular glue of the paint the way too much water will do: also helps keep the pigments suspended. You could get an equivalent effect by cutting paint with water based "satin", "matte" or "gloss" that you can find from MM or Vallejo. (They're all white and dry clear. Artist mediums simply cost about 5% what you'd pay for the others and work the same way.)
There are alternatives.
Eric