I have never been "blown away" by remastered CDs. But in all cases of the ones I have purchased, the remastered version was better. For example, I have purchased at least 3 different copies of Mozart's Horn Concerti with Dennis Brain (it has been remastered at least twice), and each time, the newer remastered version was slightly better than the earlier version.
In my experience, if you already have a version, a remastered one is only worthwhile if you have money to burn, or really like the particular disc in question. And, of course, you should hang onto your old version until after hearing the new one, in case you don't like the new one.
The biggest difference I have ever heard in remastering has been with the Beatles
Yellow Submarine Songtrack (
Let it Be...Naked was a different mix, eliminating the choir, so it goes beyond simply remastering). The
Yellow Submarine Songtrack is much clearer than the original, but, still, one needs to like the music on it more than just casually for it to be worth it, unless, of course, one has money to burn.
I suppose I should point out that some music that is very old, and is now in the public domain, has been issued by more than one company, and each mastering by different companies may differ in quality considerably. Surprisingly enough, though, it is often the case that an "off brand" does a better job with it than the original company. Here are a couple of good examples of "off brands" doing a good job:
http://www.amazon.com/Hot-Fives-Sevens-Louis-Armstrong/dp/B00001ZWLP/
http://www.amazon.com/gp/product/B00030B92K/ref=cm_cr_asin_lnk
With both of the above, they can be purchased one disc at a time, instead of in the above sets, if one wishes.
Also, some people prefer that with such old recordings, that great effort is done to remove record surface noise. Unfortunately, that always interferes with the music, so it sounds less natural when made quieter. The above sets sound as natural as you will probably ever be able to hear them, so they both contain record surface noise.