5mil lovehoney do have those pics wrong I too put smaller ring over cock and balls and pull larger ring over balls. If I remember right the instructions do show it as the correct way to wear. I love the perfect fit one. It is my favourite and have also reviewed it.
I have three stainless steel ones one's too big one's just a bit small and the middle one is just right. They do make a difference especially as you get older, sometimes I leave mine on all the time(it's so comfortable), the small one makes it really hard which is quite pleasing to see or feel but can be awarkward to remove afterwards as there is er,, more swelling. Give them a go but don't over estimate the size.
I have a few cock rings but favour the double ring ones that also cup your balls. I would not go for some that are made of a soft jelly material as I just don't think they are strong enough.
http://www.lovehoney.co.uk/product.cfm?p=22187 This is a good one.
As others have said they really do allow retention of blood in the shaft and really swell the head of your penis so that it gets a great purple sheen to it and the ridge is really that much more pronounced. It looks great. My wife loves the extra definition especially when performing oral sex.
We also have this,
http://www.lovehoney.co.uk/product.cfm?p=29673
It lacks the double ring but does have a little protrusion that rubs and stimulates the clit. Shame it hasn't got the double ring as it would be perfect.
KC - I agree that the metal ones are preferable, I especially like ones with a thicker girth. The jelly ones worn round the shaft do very little for me, but the Perfect Fit one really does the job pretty well and is good for a beginner to try.
Stuburns - it was your review that encouraged me to buy the perfect fit one! 
I love them, they work great if you find the right one, and they just help things be that little bit harder.... Personally I find these the best and they are cheap which is great, I've lost them, snapped some, left them in hotel rooms and they do tend to strech out after a while so not a big deal to replace them. Defo worth a try
http://www.lovehoney.co.uk/product.cfm?p=18275
http://www.lovehoney.co.uk/product.cfm?p=15482Cheers 5mil. Makes writing them worthwhile.
I have a silicone one which is smooth and easy to put on but dos get quite tight sometimes.
A stainless steel one which I had made for me in India - as these things are dead cheap out there so cost just under £3.00. Tryin to explain to them what I wanted was amusing as they didn't speak English and I can barely speak Indian. They ended up asking me what it was for and I just fumbled and said a friend asked me to get it!
I also recently got a leather Dominix one which is great as it has a belt buckle style fastening so adjustable and looks the business.