6mm figures have been around for some considerable time - they were actually the first metal miniatures I ever bought - Leicester Micro-tanks WW2 tanks - back in 1971 I think.
I went off WW2 gaming for a long while, partly due to lack of opponents but also having been unable to find a ruleset that did what I wanted to do - larger scale actions without the minutiae of penetration factors and armour thicknesses. My next foray into 6mm was going to be ACW but that never got beyond the "I'll get a few samples" stage - that was about the time when Baccus first introduced their ACW range.
It wasn't until I got interested in Renaissance warfare that I actually started to collect them in quantity (that was a few years ago). I still didn't paint them - I'm only just getting round to painting the majority now, for the new Maximillian rules. Since then I've done Zulu War using Heroics & Ros, HoTT armies using Irregular and I'm still working on War of the Spanish Succession using Baccus. In between I have done a fair bit from all those manufacturers for clients, including some very cute DBA armies using Rapier figures (from Newline).
Thanks to the postal strike (couldn't progress any commissions as I was waiting for figures) and a bit of painting over Xmas I have nearly completed my Maximillian armies now and the HoTT armies have grown too.
I don't think that I have the experience yet with 6mm to be able to pontificate about them like I do with 2mm but I now see them as my scale of choice. I'll still keep my 15mm Medievals - I may even paint them all one day - and may expand those to some extent but I can't see me doing any other periods in 15mm. From now on it is 6mm or smaller.