In theory, there should be very little, but I think it depends on use. Anyone who uses a computer probably knows about fragmentation and the same applies to TV recorders (though it's far less pronounced as generally on these recorders, you're talking mainly large files). Simplistically, this is where files have to be broken up into several different chunks on the hard drive to fit the file on. This can cause the odd blip in playback as the spindle in the hard drive has to to move around the disc to find the next "part" of the file it's playing. Therefore, it's probably worth formatting the drive every now and then to clear any fragmented files - every now and then in this instance being every 1-2 years as opposed to running defragmentation tools every 1-2 months on a PC!
Of course, any time a file is saved to a hard drive, there can be errors in this and thus error correction must come in to handle these, so in theory, I guess there might be a noticeable degradation in quality. However, I've never seen anything to show this in my Topfield, other than the blips I get every now and then which I attribute more to fragmentation or just a bad signal at the time of recording than to this.