I'll be honest, I didn't like either.
I'm all for zombie-type (I can't really call them 'true' zombies) but 28Days just didn't cut it for me. Good, but certainly (imo) nothing to write home about. I liked the focus on the small group though. And it was entertaining.
28 Weeks? Ha. The first time I watched it, I fell asleep halfway through (in the cinema). When i actually managed to stay awake through, I actually laughed. I found the plot to be lacking. It It was no where near as good as the first one.
28 Months? I'm looking forward to it, truth be told, if only to see what, if anything happens next. I'm hoping we manage to get a good film (on parr, if not better than Days). Paris or Russia, it could be fun.