I like the "warmer' sound with a bit of vibrato that is often missing from period instruments. There is also (at times) a whining quality I hear from period instruments.
Though I must admit, that a number of the more recent recordings i have heard by period instrument groups strike me as much better sounding than some earlier recordings.
My impression is that some more recent Period Instrument quartets have moderated their approach a bit and incorporated a modest bit of vibrato and sound less whiney.
At the same time a number of string quartets who do not identify as Period Instrument groups have also modified their playing to become more sensitive to some period instrument performance concepts.