Quality of results depends on a couple of things, mainly the brand/quality of the tape adapter you use, don't get one of the Poundland specials, go for something like this one (which is what I bought):
amazon.co.uk/gp/product/B0007R4LI8/ref=wms_ohs_product?ie=UTF8&psc=1
I can't stand listening to crap quality music in a car, but this really is spot on. As others have said, you should turn the device (i.e. phone, iPod) almost to full volume, but not the whole way (otherwise you get a bit of distortion), then just adjust the volume on the headunit like normal.
I did originally have a sony headunit in the glove box, but I wish I'd never gone to that hassle now!