Sometimes they're better, sometimes I can't tell the difference, and sometimes they just sound a little different - not necessarily better. In many cases, I think its a marketing ploy to boost music sales - issue a remastered version w/ a few extra tracks. I no longer replace favorite albums simply because a new remastered one becomes available.
Someone pointed out the Rhino remasters - those are very good. I also have the S. Hoffman remasters of 'Minute by Minute' & 'Bad Company' which are also an improvement. Many of the other 'remasters' I've compared are no better, or in the case of 'Breakfast in America', for example, sound awful. Often it sounds as if they've merely raised the recording level - older cd's I notice require the volume turned up a bit more.