I love mine.
First off a HRM consists of two parts, the watch and the chest strap. The chest strap sits pretty much directly under the bra line and against the skin (not sure how it's placed for guys !) and has a hard section for the front and an elasticised bit for the back. For all of 10seconds you notice it's there but due to the placement being an area that doesn't move I haven't ever noticed it. Weirdly enough the chest strap just stays in place.... at first I used to wet the underside to make it stick but I don't find I need to.
The watch takes about 2 minutes to set up - some are ridiculously complicated and require strides to be measured etc, the F4 doesn't and consists of putting in your censored, age, height and weight (all in complete english words). The F4 then gives you a minimum and maximum heart rate based upon age (you can change it) and if it doesn't annoy the crap out of you like it does me you can have the beeping turned on so it'll warn you when your pushing too hard or not pushing enough.
So - you have the watch on, the chest strap fastened and your ready to workout - all you do is press the button on the left side of the watch.... it then takes 2 seconds to sync with the chest strap and up comes your heart rate. With the F4 for every "session" it keeps track of calories burned, highest HR, lowest HR and AVG Hr and depending on which view you have on the watch (time, calories, HR etc - changed by pressing the right button) you have little bullets come up with every few minutes your working out in your target zone. It does totals for the week or set period.
So - whats the point of them ? for me as someone who likes running but is slow I use it to make sure I'm constantly pushing myself and therefore increasing my fitness levels. THe best bit however is looking down at the end of a workout and seeing that I've burnt 450calories or looking at the totals and see that I've done 6 sessions this week and that I've worked out for 6.2 hours and burnt 3400 calories. Its a constant reminder of the whole calories in v calories out fight.
There you go boys and girls -- waaay more information than you probably ever wanted