One of the best trips I ever did was about 3 weeks in SE Asia on an Africa Twin (way back in Nov/Dec 2005): Thailand (Bangkok) to Cambodia via Pailin, then Phnom Penh, Kratie, Stung Treng, cross into Laos, then Pakxe, Savannakhet, Vientiane, Vang Vieng, Luang Prabang, where we put the bikes on a small freight boat carrying mostly salt bags, and sailed 2 days north on the Mekong river, unloaded at Huay Xai, crossed back into Thailand, and finally headed south enjoying the great, curvy roads of the region. All 3 countries are very different from one another, maybe that´s what made it such a great trip.
Was a bit of a rush in 3 weeks, though, one could easily spend 6 weeks on that trip. Unless you´ve done them before, might surely be nice to include Angkor temples, and/or Sihanoukville and Cambodia´s coastal areas into the trip plan, as well as Si Phan Don in southern Laos. Also the route via Mae Hong Son in Thailand, close to the Burmese border, is fantastic. (But it´s worth noting, that 3 weeks will definitely not be enough to do all these!) Roads were quite tolerable everywhere (there was one 2-up bike in our group), just some roadworks in Cambodia, but I guess its whole road infrastructure should be better by now.
But I don´t know how border crossings would go with a rented bike. I did not do this on my own bike, but the bike´s owner was with us, so that surely helped a lot.
Note, that you´re very unlikely to succesfully take any other than Vietnamese registered bikes across the border into Vietnam.
edit. Vietnam: It does seem to work the other way round (Vietnamese bikes to Cambodia, Laos, Thailand and back to VN).. but your selection of bikes in VN, especially for 2-up, is probably a lot worse than in Thailand, for example.
Last edited by pecha72; 1 Sep 2012 at 20:42.