Just for some more up to date things, and some pictures, I own a Giro Targa, It has 15 VERY big vents, about 3cm by 2cm each. Its really well ventilated and reasonably light, I like helmets with Visors, as do most people, It gives the look that your helmet is thinner and more streamlined, I also like the high back, This Makes you feel although your safer because the back covers more than the top of your head unlike some of the really cheap helmets over here in the UK...
Before that i had a Specialized Chamonix, This was roughly the same as the targa but had smaller vents, but there were more of them, I also liked the feature that in the front of the helmet, underneath the visor were 3 vents, this kept your Forehead cool, which i havent seen in many other helmets... I love Helmets to be comfortable, Not too tight and not to loose, Just a snug fit..... ( As do all other cyclists probably)
Here are a few pics. contact me on
lr2188uk@hotmail.com if you want any more details.
Giro Targa -
http://www.giro.com/europe/cyc_targa.html
Specialized Chamonix -
http://mcccycles.co.uk/mcc2.asp?code=151