The quality difference is not all that great, part of the reason is that CLV is doing away with 'non-image' data, and so the reduction in space required on the disc, is not directly related to the quality reduction (i.e. taking up half the space does not equate to being half the quality).
CAV discs spin at a constant speed (RPM) , CLV changes the RPMs as the disc plays. On CAV discs you get one frame per rotation, on CLV discs between 1 and 3 frames (CAV is 1800RPM, which is 30 revs per second - NTSC is 30 fps, so that equates to 1 frame per revolution -that is why PAL and NTSC discs rotate at different speeds.PAL 25fps = 25 revs per second = 1500RPM)
CAV has a better average signal to noise ratio than CLV, around 2db better. CAV and CLV have the same SN ratio at the start of the disc, but CAV's s/n improves as you get towards the end of the disc where you get up to 2.6 times the amount of space for the pits and lands to take up.
Having to constantly change the RPMs on a big heavy disc is hard to do, so you do get TBC problems on CLV discs, especially if your player doesn't have a decent TBC built in. This manifests itself in colour problems and poor vertical edge stability. (Just like VHS - if you haven't got a TBC and you live in NTSC land, get one, you will be amazed how much better your VHS library will look.)
Other weirdnesses, errors on CAV encoded discs show up as a solid pixel or a horizontal line of pixels, CLV errors tend to have the pixels 'move' across the screen.
The main difference in consumer land is navigation - i.e. unless you have a LD player with a good digital frame store, you can't freeze frame or fast forward at multiple speeds, or jump to a frame etc.
In practice, on a good player, there is not a huge difference between CLV and CAV, often the transfer itself makes more of a difference, but with the same transfer, the CAV definately gives a better picture overall, but on some frames may be identical to the CLV version.