When I got back into wargaming a few years ago I made a conscious decision to go with 15mm rather than the more obvious scale of 28mm. I like 28mm figures a lot and have painted plenty over the years, primarily for use with role playing games. However I do find that painting 28mm figures is pretty time consuming and time is not a something I have very much of. (I have zero time in the week for painting; the only time I have is a few hours at the weekends). In practice it will probably take me at least an hour to paint most 28mm figures; even if I only want to play a skirmish game with about 30 figures a side that is a pretty big time investment. If I'd gone with 28mm I think I would probably have given up in frustration by now.
On the other hand I can probably paint about a batch of six 15mm figures in 2 to 3 hours. It becomes a lot more achievable to paint a reasonable sized force and projects become something you can see an end to instead of huge mountains to climb. The figures still look pretty good on the table even if they are not individually as good as 28mm's
With 15mm, I can get away, in many cases, with using figures for 'dual purposes'. Generally I base my figures in pairs. This works pretty well with the bigger games, which one classically thinks are designed for 15mm figures, ie at company level or higher. I find I can quite reasonably play skirmish games such as Sharp Practice with 15mm figures based this way. It may not look quite as effective as playing with 28mm figures, but it is certainly possible and we've had plenty of good games with them. I really think though that it would be pushing that too far to use 10mm figures or smaller for games like that. I've looked quite hard at 10mm figures and I know people get good results with them, but I'm doubtful I would enjoy painting them or really feel satisfied with the results.
I guess the other reason I picked on 15mm as a scale is space. I don't have access to a really large table at home; nor am I likely to have. On the other hand I could originally get going with some games on my dining table and I have now upgraded to a 6'x4' table which can accommodate pretty respectable sized games in 15mm.
Finally, and most recently, I thought about whether to work in a larger scale for small projects. For example I could go with 28mm for say TFL's upcoming Chain of Command rules which is really what they are intended for. However, I realised that having built up a collection of terrain for 15mm, the last thing I want to do is start another collection of bigger terrain; those 28mm houses are pretty big and take up a lot of storage.
So I'm sticking with 15mm as my scale of choice. There are some great ranges of figures out there; let's hope it stays that way. I'll finish with a couple of pics of fairly new napoleonics from my collection; they have yet to see any action.