Choosing between Reissue and Original pressings

Reissues have been common since quite some time but these days as vinyl has picked up momentum again, there have been some very high quality reissues from labels like Classic, Analogue Productions, Speakers corner, MFSL, Boxstar etc. For any particular album how do you decide whether buy a good reissue or get a good copy of original pressing from ebay ?

For the sake of discussion lets keep out exceptions where the original pressing is too hard to get or too expensive. In most cases it is possible to buy a copy of original pressing for sane amount of money if one shops carefully on ebay but I have also found that quite a few times a high quality reissue can sound better. Whats the general thought among hardcore vinyl followers here ?