You'll get as many opinions on this as there are people out there!
It depends completely on how well you ride, where you ride, how fast/hard you insist on riding, and where you're interested in going. People have done Africa successfully on Gold Wings, Yamaha R1's, (Sjaak Lucassen) and full dress Harleys - see the
Forwoods story.
IF you ride with your friend regularly, and ride similarly and as well, AND want to go to all the places he's been, then pay attention, his comments are very valid for you. But I suspect you're different from him...
For really serious, nasty off-road, there are better bikes - BUT the GS will DO IT. Perhaps not as quickly as say a KTM 640 Adventure, or as cheaply as an XT600 CAN be, or as comfortably as a Gold Wing,

but certainly just fine. AND probably more reliably than an XT. Chance are very high that the GS will not breakdown at all, whereas an old average XT probably will. When the XT does break, yes it's easy to fix - but you still have to know what you're doing, and you may not have the right parts etc. A really well-prepped - by an expert - XT is not cheap, but is reliable.
Most problems the GS is likley to have are as easy to fix as the XT. If the electronics fail, or the fuel injection, yes it's a pain, but you can have replacement parts shipped in easily. AND most importantly, they are EXTREMELY unlikely to cause ANY trouble at all.
My advice is to sort what you have, and ride it. Be sure you really need the big tank too. A spare plastic fuel can lashed on somewhere can do as well, and most of the time it's empty and weighs little. And it's really cheap.
Pierre Saslawsky and Merritt Grooms just rode a pair of F650GS's down through Africa. Search the site for "Pierre Saslawsky" for lots of hits and info on their experiences.
Have a great ride!
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Grant Johnson
Seek, and ye shall find.
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One world, Two wheels.
www.HorizonsUnlimited.com