Just to chime in (mostly) with what has already been said. VW is the best management of batteries there can be. The first PVs were just a battery and an atty/carto and a switch. There are many variations but at the end of the day, they are all basically the same. PWM is a side improvement, that gave better battery life. The next big step is variable voltage. For the first time, you can tailor the hit to what you want without having to swap parts out to do it. There are many different ways to actually do VV, but the end user result is the same. Variable wattage automatically adjusts the voltage based on the resistance of your atty/carto, your last puff is exactly like the first puff.
Tanks are not good on a Darwin if you want it pocketable, but there are several large/long cartos that hold 3-5 ml of juice. If you don't need to put it in your pocket, tanks are great on a Darwin.
Some do not like the fixed batteries, but I consider it a plus. It always lasts more than I can stay awake, and in my life, I am near power every time I sleep. The ability to plug it in and vape while charging is great, and especially well suited for anyone that spends a lot of time in a vehicle. For my use I do not prefer a separate charger and multiple batteries, and routine disassembly of the device just in normal use. Yes, if the battery fails you are without the device, but that's what backups are for. If you lose it or it is stolen, it is just as gone, keep your last device as a backup!
Battery technology has made huge leaps and bounds in the last 5 years, driven by the boom in cellphones/laptops/netbooks/MyPad devices. Full 2 year warranty on the Darwin and a 1 year battery warranty, I believe the battery is $25 out of warranty.
When I first started vaping, everywhere I went I took spare batteries, bottles of juice, paper clips, multiple atty/cartos, napkins, chargers, and a PT. I may not have had to, but I kept a charger at work, a charger at home, and one in the vehicle. With Darwin my routine is much simpler. I keep a few cartos in my vehicle and at work, and one 3 ml bottle of juice lasts me all day.
There will continue to be improvements in vaping technology; better feeders, more effective battery life, bluetooth MP3 players, MySpaceBook connections, GPS, more display and menu choices and smaller sizes. The form factor of Darwin is not for some, but performance-wise, it is as good as it gets right now.
EDIT: There are of course, all types. If you like to vape at 10 volts, your (really) big-batt mod will be best for you. Darwin's performance is as good as it gets for most people.