I know that at this moment, which is still too early to say anything difinitive for my own experience... I'm not having any problems to speak of though on the v1 VN tube using a v2.5 head I have a concern, but no problem really.
First... the new all v2.5 VN. I've just started on my second tankfull in it. Both the flavor as well as the amount of vapor have steadly increased. Unlike Briar, I've only used one flavor ( a RY4 )... so I can't do a nice comparison as she did on the nuances in this area. In fact, I normally only vape 3 flavors ( all tobacco based ) and only venture into deserts, fruits and drink flavors when the mood strikes which is a bit rare for me.
Just to rub it in a bit more ( because it's fun 

 ).... I have no idea if my heads ohm out even close to their marked rating... I have a Kick and don't have to worry about it since the Kick adjusts it for me automatically to the wattage I want. ( Don't ya just hate those people that gloat ... lol ). Now to get back to reality again...
On the v1 VN with v2.5 heads... darn it... it is still an airy draw ( duh ). Of course I knew it would be and if I cared enough, I could address that if I really wanted. I can say that my results on it are very similar to the v2.5 VN but I can't compare flavor much since I'm using a different tobacco that has a mild but complex flavor in it... no carmel or vanilla like the RY4. I have almost completely ( but not quite ) vaped a full tank in it. When I first filled it... I laid it on it's side to see if it was prone to leaking that way... no leaks were found. The acid test for leaks will come when I'm taking it to work, carrying it in a hot car etc... so those results will have to wait. 
The concern I mentioned, and I'm uncertain if this indicates a problem or not... is that it looks like inside the tank is condensing... I don't recall seeing this on the all v2.5 tank. I also can't say if it even a problem really. I only suspect that the top seal isn't as good as on the v2.5 tank but that is only a guess at this point. I did also have a few moments where I got vapor into the tank and I readjusted the top seal a second time to eliminate that. 
Things are certainly looking good for the v2.5 all in all at this time and right now I'm very happy ( but reserve the right to flame bad and danno at any moment ) ...
Sev... listen to what Briar said... I second it for what it's worth. I know... it's hard for you young whippersnappers to listen to us more... eh... experienced folk ( just to stay away from giving away my own age ) 
