incompatibility issues & not working up to their specifications are the two main problems i have encountered w/ generic ram. not all of them are like this, but a majority are, making it worth it to just spend the extra bucks to assure you'll get what you need.
edit - also, if you read the posts on here, you'll notice that alot of folks who buy name brand ram (kingston, mushkin) usually end up getting more than their money's worth as they are able to run it up to speeds of 2-2-2 133+ when its only specified as 3-3-3.