I've got one on my bike. If you don't already have progressive front springs, you will for this tank.

Advantages:
Plastic tank- can take quite a beating without issues.
Plenty of range
Easier to remove tank than standard tank with that stupid valve at the back
Excellent wind-shield for your legs.
Looks cool.


Disadvantages:
Cost compared to other solutions, i.e. carrying two jerry cans mounted on the crash bars lower down
Bit more top heavy
Does restrict airflow through radiators a bit, especially if you have tankbags hanging down, e.g. VP45 Touratech type
Lot more difficult to get to engine without stripping tank off.
When going downhill (such as mountain passes), the last 5-7 liters of fuel sits in the front of the tank and the fuel-taps are above the the fuel line
Hard to find crash-bars that fit around the tank- think only Touratech makes some and be prepared to make your own or modify.
Other bits, such as Boano fairing not compatible with big tank and be prepared to modify.

As for fuel-availability- can't speak for everywhere, but places where you might have issues with a fuel range <400-600km include Angola, Botswana, Mozambique and Zimbabwe (especially of the main roads, which I think you were asking about). You'll get some fuel on the black market, but quality is dubious and it's an adventure to find fuel.

My AT with standard tank and jerry cans




My AT with big tank and standard fairings
http://i692.photobucket.com/albums/v...3596Medium.jpg

My AT with big tank, Boana fairings








So in the final analysis- do I think the big tank was worth it?

I will never buy one of these new from Boano, Touratech or Africa Queens- just too expensive. But now that I have it, I don't think I'll ever go back to the standard tank (which I still have). Only time you have too much fuel, is when you're on fire. I sometimes still pack an additional 10liters of fuel- we live remote.

And you get used to the weight and the feel of the bike. One advantage of the jerry-can system though: the bike lies at a convenient angle for easier picking up (and thus the bike is easier to handle compared to the big tank)

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Squily