I use pretty much only USB2 and even USB1 externals. Never had a read/write problem unless a drive was a goner. I had for a while over 900KB/s down. My ex had a 20Mbit line (almost 2.5MB/s) maxed a lot of the time to a USB2 external. Never had a problem.

When I move stuff between USB2 drives it's usually at a speed ranging between 15 and 23MB/s and unless you run a GB box and have nice axx, there is no way in hell you get close to those speeds just downloading.

The AntiVirus predicament Peerless mentions may be valid in some peoples cases although I never noticed it myself. Guess it depends how you're AV is set up and what type it is.

As for shared files disappearing. I have no idea unless your drive changes drive letter on you. But I would think the fluctuations would be much more if that was the case.