If you want lightweight and the 510 the VP1 or VP2 are good. The power is pretty much the same (throat hit-wise and flavor-wise) as the factory batteries, they just last way way way longer. I used my VP2 last night for over 6 hours and it was still going when I switched batteries because I try to never run them down all the way, since that is bad for Li-Ion batteries. I got the VP2 because it uses protected batteries. The ring of LEDs are very pretty, and the unit works like a dream. (can be used with the 801, or you can get 901 or 510 adapters--I have the 510 adapter)
For high voltage I have the Adaptever. I use it at 3.7v which is the same power level as the factory batteries. I also use it with the high voltage sleeve to use 6v. That gives a stronger hit if you draw for the same amount of time as you do on the factory batteries, or if you want the same hit that takes a 6 second draw on the factory ones it only takes 2 secs on the 6v. It is better for me when I want to take a few puffs and set it down for 30 mins. It also works better for the strong, heavy flavors than it does for the light fruity tastes. The Adaptever is heavier, but it is comfortable to hold and is very sturdy. Can be ordered to fit 801, 901, or 510 and you can get adapters to switch out if you have more than one kind of atty.
Both are great and I use whichever suits my needs at the time.