I rode the Trans-Siberian Highway last summer (2005) and didn't find it as terrible or difficult to ride as some have described. I have a day by day description of this ride, with many pictures, on my website. The so-called Zilow Gap is the largely unpaved section from Khabarovsk (actually, the pavement ends west of Birobidzhan) to somewhat east of Chita. This section has been under construction for some time, and the Russian government hopes to have it all paved by 2008. It was not possible to drive or ride this route completely across Siberia at all (except possibly in winter when everything is frozen) before about 3-4 years ago, when the government began the construction project.
Much of this road is now wide, hard-pack with an overlying thin layer of gravel, apparently completed except for paving, and easily ridden at some speed - 50 mph or more if one is comfortable riding off-pavement. Some is still (as of 2005) under construction with one lane being used for traffic, but largely easily ridden. Some is still local road that varies widely in condition, from easy to fairly bad. And in some spots the road bed is still being contructed and one rides over what is the base layer of road bed - large loose stones, etc. The area just east and west of the intersection with the north-south road from Skovorodino to Tynda was the worst of this type (it was called a "slag heap" by one rider). I had no difficulty with tires being punctured by stones, etc. at any time. Each summer there is more construction activity and I assume each year this road becomes easier to ride or drive. Gasoline is readily availble in most areas, and there are cafes at (fairly wide) intervals.
This area has been described as very swampy by some, but I didn't see that -never noticed a swamp of any size. Perhaps that was because I went in late summer (last two weeks of August), and earlier in the summer and closer to the snow-melt it might have been wetter. It is largely forested, with some grassy meadows interspersed. One of the advantages of going as late in the summer as I did is that the nights were already quite cool (high 30s - low 40s F) and that resulted in far less of a mosquito and other biting insects than many travelers have reported. The days were mostly sunny, warm, and very pleasant for riding. I encountered essentially no rain in this section (although I did further west in Russia), resulting in quite a lot of dust in some areas.
It seems a riders perception of riding the Gap depends a great deal on his or her level of dirt riding experience (I have a good many years experience). For those experienced in dirt it is not a difficult ride, but for those with no dirt experience it can be intimidating. I would recommend that anyone with no dirt riding experience who is planning on doing this route practice off-highway riding as much as possible before departing - they don't need to go to a motocross or ohv park, just find some gravel or dirt roads to practice on. All sorts of bikes have done this route, from Gold Wings to dirt bikes. It isn't necessary to have a dirt bike, but one designed for a bit of off-pavement use would make the ride easier and more enjoyable than would be the case with a heavy touring bike, I would think.
I thoroughly enjoyed this part of my rtw ride, especially since I had three riding companions for part of it, three young Russian riders I met along the way. I would advise anyone comtemplating doing this route to not be intimidated by the stories sometimes heard or read, but to go for it!