Cheap generic batteries and name-brand batteries have basically the same formulation so they lasts about the same amount of time. In fact, Consumer Reports gave top ratings to a store brand for value. The problem is that cheaper batteries are cheaply made, meaning tolerances aren't as great and construction is mediocre. That means leakage is a risk. See the Kodak re-branded alkaline batteries below, which are made in China. Kodak, of course, doesn't make batteries. Neither does Polaroid, Sanyo, Fuji, Panasonic or the many other electronics companies that sell batteries with their name.
These batteries were dirt cheap, at $5.99 for 48. Although the expiration date is Dec 2011, the batteries began leaking in storage months ago (in a cool dark dry room). In the sample battery I took out, both the anode(+) and cathode(-) are leaking. About 20% of the batteries have visible leakage, and every battery has a powdery coating. Many, however, still tests around 1.5V (aka a fresh battery) but would you risk ruining your equipment by putting these in? I'm putting everything in the trash.
Major battery manufacturers also make industrial versions which cost a lot more. They're usually sold to the police, military, etc. My hospital uses them. Their specs are identical to normal alkalines. They don't last longer or have a higher voltage. But the casing is much stronger, and that's what you're paying for. And that's also the comparison between name brands (i.e. Eveready, Duracell, Rayovac) and generic batteries (or in some cases, rebranded nes like Sanyo, Kodak, Polaroid, etc). See Duracell example below.
BTW, in my experience, Rayovacs are more prone to leakage than other name brands, which explains why they're cheaper. Then again, I torture my alkaline batteries, i.e. I trickle-recharge them using a special charger.