Whats the m401 like? Maybe I will get one of those for when I'm driving rather than an auto battery for my 510 that I just bought.
If you've already got 510 stuff and like it and how it works (size, cart filling, vapor, flavor and such), might as well spring for 510 auto batts for it rather than getting the whole schemeal (attys,chargers,cases,etc) for 40x stuff too.
I've only used m40x stuff, so I've got a very limited point of view and some of what i'm about to type comes from reading this forum rather than first hand experience... but to answer "whats the m401 like" question...
Its a very efficient ecig model. The batteries tend to last longer than others, it seems to me because the system (batt+atty) is arranged to consume less power than some other models. This also means it runs thru liquid more slowly, and the cart size is somewhat smaller than other models (too small really, but aren't they all). But that efficiency comes at a cost. It doesn't produce as much vapor as some other models. Then again, it doesn't scorch or burn liquid as easily as some other models either. I've read that the 401 is better for flavor while the 510 is better for vapor and throat hit. (I suspect thats pretty much a function of power consumption). I've read that some people prefer the 401 for fruit/desert flavors and the 510 for tobacco/coffee flavors.
It has an exposed bridge atomizer design. While this is not so good for "dripping" (you can drip but its more tedious), its perfect for "dipping" (which pretty much can't be done without an exposed bridge). I use a contact lens case for dipping.
The battery size options are pretty nice: small (403), medium(402) and large(401). The smallest is the same size as an analog cig. The medium, slightly smaller than a 100. The largest being 108mm long. There is a nice PCC available for it (in two sizes one for the 401/402 and another for the 402/403).
My favorite to use is the 403 really... so small and light.
Parts are readily available from a lot of online retailers, including "XL" carts.
One thing that bugs me is how few MODs have an option to use the m40x atty on it. Of course there are thread adapters that can remedy that.
If I had to do it all over again, I might start with the KR808D model and cartomizers for the larger capacity and convenience of not having to refill carts as often or clean attys. I'm going to get a Chameleon soon, after that I'll finally give other attys and cartomizers a whirl.