I would argue that 'sound good' is relative (and based on use/need/cost)..
If your friend has them hooked up to a stereo (in a car mind you), see if you can listen to it while playing some of YOUR music.. see if you like it.. try to see what he's using for amps/EQs/etc..
Just remeber that it won't sound exactly the same in your car, but you can at least see if they will meet your needs.. just don't pay too much for them..
If not, see if he will let you hook it up in your car to see what it sounds like (this presuming you will get subs anyway, even if you don't like his //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif )