I work in a shop in Winter Park, CO and we sell quite a few of these, and a good number of people I know use them.
I recommend the Hotronic m4 or m3 ... I got the m3 for my mom, who has extremely poor circulation in her feet and usually can only ski for an hour or two even on a warm day, now she has no problems all day.
With the setting on 1 out of 4, still 90 degrees or so, the batteries last for several days.
They're not cheap, almost $200, but if you have chronically cold feet, it's worth it.
As far as the disposable heat packs go, if your boots fit properly, you shouldn't be able to fit one inside your boot. And if you get the heaters, someone at the shop should be able to install them for free if you're at a decent shop