Depending on the evenness of the ground, there is a difference in height between the tracks, which results in a bump which you can both hear and feel as a set of wheels passes over the junction. The impact on the shot is minimal since the difference amounts to the thickness of a sheet of paper (or several). This is my experience with the Glideshot system (around $760 for 15 ft of track plus a heavy duty dolly).

If you record audio during a dolly shot, and the mic picks up the sound, how do you get rid of it? Are all dolly shots put through ADR or noise fixup? Or are the professional dolly tracks built so well that the track-to-track junction is absolutely seamless?

Whatever, a dolly system is a great acquisition. You can do a 12 ft moving shot much more smoothly than walking a camera along the same path.

Marc:

High-end dolly tracks are precision built out of steel (sometimes aluminum) and have extremely smooth joints. The trick is that wooden wedges are used to make sure they are perfectly level especially at the seams. Spirit levels are used to assure this. A long dolly run over uneven terrain takes several skilled grips and a reasonable amount of time to level and trouble-shoot for bumps. If all is done properly, their operation is silent. The use of "skate wheels" will aid in this also.