Well, the 1st movie I didn't really give a crap about it and I avoided it ever since it came out. I wasn't expecting too much from it when I bought it, but I was surprised after watching it that it turned out to be a pretty decent film. The 2nd one looked awesome in the trailer, but it kind of failed to live up to my expectations, and the camerawork was too fast paced in the action scenes.
But if you're really a fan of zombie/apocalyptic movies like Resident Evil and if you can get both films for $20 or less, buy it, otherwise just rent it. If you're looking to buy, I'd say wait until 28 Weeks Later drops to $10 or less. 28 Days Later is already under $10 so that way you can get both films for under $20, unless you really need to have that cheap cardboard box to put them in which is the only extra incentive for buying the combo set.