i did a big test in a store, shoot-out between a lot of preamp/compressors. ended up getting the UA-LA610, just had that "special" quality about it. and this was before i brought a UAD card!! Its hard to explain, the other preamps just sounded "flat" and lifeless. The LA610 sounded hott!! volume levels were the same. it was like the LA610 sounded like the original and the other preamps (some more expensive) sounded like 2nd gen tape copies. everyone in the store, including the employees, all gathered around at the end and all agreed, and were quite amazed at the difference. Most didnt expect such a difference between them all. It was very noticeable. This was up against a DBX preamp, Neve and several others more expensive (i forget which ones now but big brands).
Then again i had an older pro-session singer in my studio a year ago and he was floored by how good my LA610 sounded. he thought i musta just got lucky with a "special" unit that has more "specialness" than most of the others...he had used lots of them. So maybe thats true, the store that day only had one unit, the display one, so i couldnt put it up against another LA610 anyways to know... they asked if i wanted to wait a few days to get one "in the box" and from experience i always take the one that i can see working and know its solid. Possibly a great decision.
Its like anything i guess. Take 5 acoustic guitars from the same model and manufacturer and year and play them after each other and one will stand out. Even if the manufacturer has high quality control, no two are really ever the same.
Needless to say, when i moved the studio a month ago, most things got packed into boxes and moved by the storage guys into the truck, the LA610 was carried like a delicate baby out of the rack and onto my car passenger seat and travelled first class

when i track vocals it just takes the rough off the edges...very little compression. As XAXAU says the compression happens later with the plugins LA2A , 1176 LN, Waves De Esser, UAD Prec Limiter etc...