My 510 reached me 2 days back and i've been using it exclusively. I only ordered the standard starter kit with the normal batteries (not manual switch) so that the comparison is fairer. Here are my comparison with the M401/2:
General Impressions
IMHO, the 510 basically is a combination of 901 and 401 in its appearance. I got a white unit. After using the Mseries for some time, i was not quite used to the design of the old 901 type, where the atomiser is long. This aside, it does look quite elegant in all white.
The M401/2 is slimmer and definitely appeals to the newbies more than the 510, in terms of appearance.
Battery
It really packs a punch out of the box. The battery seems to deliver much more power to the atomiser. It seems like an overclocked battery to me. Besides that, the battery works exactly the same as any Mseries battery. With this being said, the battery life is really short. Really, really short. It requires charging in less than 1 hour of continuous and heavy vaping. The M401 wins hands down.
Atomiser
The atomiser resembles that of a 901. However, the steel bridge is pointy and extends further down than a 901. The steel part is also much smaller than the Mseries.
The performance of a 510 out of the box is excellent in my opinion. As mentioned above, i suspect that the battery is designed to deliver much more power to the atomiser compared to other models. There does not seem to be a break in period needed for the 510.
However, only after TWO days of continuous use, the atomiser is starting to weaken significantly. In the beginning, i only need to take very short and light puffs to get a good hit. Now, even long puffs will not create the type of hit i got 2 days ago. I think the extra power is taking its toll on the atomiser. I can't help but feel that if the atomiser steel bridge is bigger, it would perhaps lasts longer. I am saying this because it is really small.
The 510 atomiser performance for the first two days would literally murder the M401/2. However, this does not last long. To make sure that my mind was not playing tricks on me, i popped in a new 510 atomiser to judge the difference. For those of you who don't know, the 510 comes with 2 atomisers. The second atomiser gave me an excellent hit. Just like what i got 2 days back. A weakened 510 atomiser is very similar to the performance of a weakened 901 atomiser.
Conclusion
I'm sticking to the Mseries for now until the 510 can work on a more lasting atomiser.
This is only my personal review. I'm interested to see ruuku's review on the 510 as well.