Mercury Living Presence

Mercury has been one of the prized classical music recording labels since it's first monoraural release in the early '50s. Here are two articles that will give you a lot of the background about this label:

http://www.soundfountain.com/amb/mercury.html
http://microgroove.jp/mercury/LivingPresence.shtml

The original vinyl is highly prized by many collectors. Speakers Corner is in the process of reissuing much of the Mercury catalog on vinyl, and the results thus far have been outstanding.
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I agree that they're quite sonically special. Try the Rachmaninoff Piano Concertos 2-3 with Byron Janis, exceptional performance.

The vinyl is equally, or even surpassingly good with the right kit.

I like them, they are IMO dynamic but bright compared to many other high quality issues.

As Rushton noted, the originals are highly prized by collectors. A fine example of a very simple recording philosophy (3 mikes across the front of an orchestra) superbly executed by a producer with excellent musical taste. The recordings have a lot of the energy you might hear at a live performance (their reproduction of orchestral dynamics was probably the best of the era), but on the whole are on the bright side, as Chadlnliz points out; the first set of SACD reissues actually seemed to tame that brightness just a little, compared to the original CDs. The CDs were remastered under the direction and direct supervision of Wilma Cozart Fine, the original producer of the records, so they had a lot of care taken in their transfer.

Glad to hear from Rushton that the LPs are worth buying, I have held off, as a number of previous re-issues of some Mercury recordings, both on Mercury's budget labels and on the Philips label, have been either disappointing or a mixed bag (not the case, though, with the few Classic Records reissues, wehich are stunning).