Quiet, quiet, quiet...! These are about as silent as an external pump is going to get... They are also very compact and have a nice small footprint...
However, they do run hot, so if your system runs hot, and you have to work at keeping it cool, these pumps are not the wisest choice...
Personally, my apartment is always around 70F, so I utilized the Velocity to actually help me heat the water, to help the heater from being on constantly. I would say these pumps added 2-3F to my tank at worst... I have heard some say 5F, but that was not my experience...
I also found my Velocity to be very reliable in the two years that I used it...