It scares me when people use them..... yes they work.... but in gusty conditions they are dangerous..... and in small sailing sites like car parks they can be downright B###DY LEATHAL.
I would seriously say for newcomers to the sport DO NOT USE CLEATS for the main sheet, a better option is make your final pulley a ratchet pulley it has all the advantages of a cleat on the main sheet and also it has the safety feature of being able to be let out at any time..
Its not a good feeling trying to get a yacht and pilot out that's wedged under a car because they couldn't let the main sheet off and panicked.

www.ronstan.com/marine/supportoverview.asp

Yachts like the Blokart, Manta, Old style Class5 or yachts with mast support bracing from the rear axle, it's often good to grip that bar and main sheet together to take some of the rope pressure.

cam cleats can be great if they are in just the right spot,and cant inadvertantly catch the sheet.For me ,i found that it was the 1 thing I liked about the blokart handlebars.
I do find that the windrange where they can be effectively used is suprisingly small. all that applies to my mini
On my 5 it is one of the things on my shopping list. when I built susans promo back in 2007 I found that with a torsional class 5 chassis it was quite reasonable to sail,even in stronger winds with the sheet cleated.It comes down to the design and your experience level.

OK Gizmo, good advise, I'll mount it, but use it when conditions say so. I'm out there to have fun, not get hurt

When I first used them they were called Jam Cleats and I was an inovator in there use in Hang Gliding in NZ. Used at the Wing Tip to retain the Sail tension.
2 Ribbed Jaws that the rope caught in. It takes very little effort to flick the Sheet away from the Cleat, one just has to be ready to release it when required.
I guess with a lot of use Familiarity Breads Contempt. They definately cann't be used in small tight confined spaces. One just doesn't have time to settle into a run before spilling to sloe and turn.
Ron

Fixed Jaws. I never use them without the Sheet firmly in my fist for a quick flick of the wrist to release it.
Ron
PS; I tend to use it when a rest is needed.