I currently use a Compaq Laptop, Presario CQ50. I've had this laptop for about 2 years and 4 months now. I am on the third battery now on this laptop. The earlier 2 batteries were so bad that they don’t provide backup for 10 minutes before the laptop goes off without warning. (This to me is actually bad as my previous laptop was an Acer Centrino Travelmate 290 which I used with the original battery for almost 4 years. Even now the battery still lasts about 30 minutes.)
I bought the third battery about 10 weeks ago, this time going for a 12 Cells 8800mAh/95Wh High Capacity Battery HSTNN-IB72. At the local store where I bought this they had 8hrs pasted on the carton. I knew this is not likely true as the 4400mAh battery that came with the laptop barely lasted 2 hours new. Several reviewers online mention the fact that the battery sometimes fall out and it also makes the laptop too heavy and that the battery lasts about 4 hours.
I found from my experience that if I totally drain the battery then it would last about 5 hours. The battery has actually fallen off on one occasion. But I find it hard to replicate by shaking the laptop. The heaviness of the battery is actually insignificant to me as the ability to keep on working easily negates whatever weight the battery adds to my backpack. I think I can say I am reasonably satisfied with the battery.
One other thing mentioned in reviews online was that the battery usually dies just after 1 year that the warranty expires. I am waiting for the 1 year mark, and I hope I won't find this true!