40-42mm seems to be a traditional toy soldier scale, but has been overwhelmingly overtaken by 28mm, which takes up less of a footprint in large battle games. Lately, skirmish games have made somewhat of a resurgence. Even still, 28mm (and the more "heroic" 33-35mm) figures are here to stay and will probably remain popular for years to come. So why do I prefer 42mm?
Miniatures companies produce some beautiful 28mm figures, many of which I still paint, but I prefer a larger canvas, and 42mm figures provide that. Being easier to paint adds to a more relaxing experience. The larger size also allows me to try out some sculpting conversions, and scenic base ideas that would have otherwise been too fiddly or difficult at a smaller scale. I've improved, and I've ported over some of those conversion skills to my 28mm stuff, but it might never have happened if I hadn't tried it out on my 42mm (and 54mm) figures first.
My friends aren't the biggest fans of minis game (they prefer the board variety,) but after discovering Two Hour Wargames (and an excellent solo game in the "Red Sands" series,) the samurai are great to play skirmish games with. And when they are on the table with large trees and rocks and other terrain, they are -- just like I said -- just damn pretty!
How about some Pros and Cons for 40-42mm figuresPros
-- Easy to paint
-- Easy to convert
-- Look gorgeous on the table (especially at conventions if you're trying to woo new players)
-- Larger scale does not come with a larger price; some 42s cost just as much per figure as 28s.
-- A good selection of figures across time periods (Ancients, Samurai, Medieval, Vikings, European wars, Pirates, AWI, Civil War, Old West, Moderns come to mind right away)
-- Great for skirmish games
-- Need extra space for large battles
-- Few players use 40mm for mainstream games/Difficult to find opponents
-- Many periods are represented, but ranges are limited (due to limited popularity)
-- Limited available terrain on the market (especially buildings)
Final word: If you like skirmish games and don't have an extensive collection of 28s yet to play, I recommend you check out what's out there for 40-42mm figures. Or if you have some shelf space, try out a few 42mm figures just to paint for fun. You can use the more relaxing experience as sort of a mental cleanser from painting smaller figures, and a few 42s will look nice (and will be noticeable from a distance) in a living room or library.
(I didn't include 54mm figures, because most are much too expensive to buy enough for a game, though, there are plenty of 54mm plastic figures out there. Bronze Age Miniatures does put out a few that are not too costly. And yes, I do love my 54mm post-apocalypse figures, too.)