In my current gaming group we have 6mm, 15mm, and 25mm Napoleonic collections, and I have seen the new Victrix and Perry's 28mm plastic miniatures. I have been tempted to get a few boxes. If not Napoleonic, then American Civil War. Though, the figure lines are not exclusively plastic (most seem to be metal), and you have to eventually get metal figures.
I think 15mm figures are the most popular because:
1) At a distance, visually they still look good (1).
2) Require less space to play a larger game.
3) Because of the smaller game table, easier to reach across it.
4) Are supposed to be easier to paint.
5) Cost less, therefor you can get more for your buck.
6) They are Easier to transport (an important factor for some).
7) Take up less space when storing away.
8) Are lighter to carry than the same number of units in 25mm scale (not plastic).
9) Are bigger than 6mm figures :-)
(1) Especially the larger 15's (true 17mm, 18mm, 19mm figs). It's amazing that some figure manufactures (and even gamers) still call an 18mm or 19mm figure a 15mm one. That's like saying a 30mm figure is a 25mm one... but people do that too :-) AB, which are some very nice figures BTW, used to call their 18mm figures 15mm ones. But now they refer to them as 18mm figures. A trend I hope spreads, because for a new comer to the hobby, this whole "large 15mm" concept can get a bit confusing I would think.
Some companies try to justify this by saying the measurement is from the ankle to the eyes, or something silly like that. But the true distance, and a lot of early gaming miniatures used this as a standard, is to measure the distance from the bottom of the feet to the top of the head excluding base and shako. And it makes sense too. When you go to the doctor, he doesn't check your height from your ankle/shin/knee to your eyes/nose/mouth, he checks your height from the bottom of your feet to the top of your head, like normal.