Since when do sequels have to be neccesary to be good? Terminator 2 didnt even follow the proper timeline, let alone events, of the first one and it still rocked.
A better comparison might to be what Aliens was compared to Alien, while not quite as drastic a change between Day and Weeks, it was a similar change in concept and scale. Days was the smaller, more tense, scary, and atmospheric piece, while Weeks is the larger, more explosion and violence filled actiony bigger brother.
To be honest I didnt like 28 Days Later because I thought the second half was garbage that took the good atmosphere of the beginning and threw it out the window. I liked 28 Weeks later because while the middle is calmer than the beginning and end, it is by and far a much more entertaining movie to watch. The graphics were done well whether it was the CG (which there wasnt much of) or the actual physical gore. Its a bit more over the top but honestly it never drags down thanks to some wonderful scenes.
The beginning segment was an excellent tense, edge of your seat, roller coaster ride and the rest didnt let down in my opinion.