John, heres an honest opinion from someone who,s had them all, and has now got another Tenere
650 GS not a bad bike but gutless no soul good on petrol till you need to get somewhare fast then your wringing its neck to get there, kudos of it being a BMW, but then most GSers feel its a bike for the wife!! only thing that went on mine was the head bearings, not a lot you can do to get any more grunt from it either as far as im aware
Tenere I bought my first one simply loved the look so it was a must have fitted TKC 80s to it took ot to Wales attempted some offroading deep muddy rutted tracks, thought it was absolutly useless the forks felt like they were attached to the handlebars by bunjee straps, front wheel going everywhere but the handlebars straight,this maybe why people on the XT660.com shange them, brought it back and sold it, the trip to Wales war Excellent you can sit at 85 no problems returning good MPG but I hated it offroad,
HOWEVER my mate who bought the same bike at the same time and did the same trip thought the same, and instead of getting rid he put a pair of MT21 tyres on his, I rode it offroad and its a different beast alltogether it felt so much better with the MT21s on it, my latest Tenere has them fitted and must say they are great and I love the bike now,
Wait for it However !!
After selling the Tenere I bought a 59 plate BMW 800 GS that I fitted MT21s on straight away used for w weekend offroading in Thetford and the bike was amazing it was simply superb faster than both the Tenere and the 650 GS cruising speed much higher tons of grunt compaired with the other 2 for overtaking and generally so much better, I loved it but the cost was putting me off for the offroading side of it as slinging an £8k bike down a muddy trail wasnt my idea of having a good time so that was sold and another Tenere was found
My current Bike has a few add ons Leo Vince Cans, DNA stage 3 filter kit, Snorkel removed, kev mod, MT21s, so on the performance is slightly better than STD but its very smooth now with the injection problem almost eliminated,
Now My advice
If money is not an issue Buy the 800GS its by far the better bike for all occasions,
If it is buy, the Tenere as its a capable bike with the right tyres and bits fitted and it is true Value for money especially if you buy a almost new one
£4k will buy you a really low mile mint bike (Half the price of the 800GS) thats good value and the smiles for miles factor is high with a value for money bike if you remember your not going to beat a WR450 offroad or a 1200GS onroad youll be fine with the Tenere