This is the violin I just got to play on Thursday: a Guerneri del Gesu from 1734 that was owned, among others, by a pretty famous modern violinist for about 10 years.
It was an amazing experience, but obviously hard to describe. The instrument is beautiful as a work of art, but it is when it sings out that one becomes entranced. Even under my unsophisticted fingers, it had a power and sweetness that makes one understand why this master maker's work is sought after by almost every great violinist since the early 1800's. The more sound you try to produce, the more reserves the instrument taps, filling the room. With all its power, though, the sound is also sweet and golden and pure--impossible to convey!
Here's a picture of the back of the same instrument. It's just possible to see, on the left, some of the original varnish, and the lines in the wood (the "curl") that show it was an expensive piece of maple for Guarneri to use. Did he use it because he was selling it to a wealthier patron?
The other picture is of a violin made by one of Antonio Stradivari's sons, whose name was Omobono. It's actually quite touching, this project the aging master gave to his younger son to complete. Compare the plain wood of the back to that of the Guarneri: vertical grain, not very pronounced or picturesque, not much character--a cheaper plank for a student's use.

It is still a beautiful violin and sounds quite good, with some of the father's sweet sound coming through. Omobono also seems to have used varnish like that of his father: the same recipe, or a pot the old teacher had made himself? The label inside the violin carefully documents, for this relatively unknown luthier, that the violin was made "under the discipline of Antonio Stradivari."
All right, so this is geeky, but it is still thrilling to hold these beautiful sojourners through history in one's hands, to reflect on the times of turmoil they have survived since being sent off into the world. If they can pass through the maelstrom, why then perhaps we can, too.
Of course, interacting with something that has such monetary value is, we might say, the guilty pleasure part of this, but there is more: across all the years, these bits of carved and varnished wood testify to the enduring powers of human genius, to the creativity our finer angels can summon from us, to the fact that the transmutation of passion and rigorous discipline into tangible beauty is a part of our common heritage.