Having just re-watched it, i have to say that while it is a good film, i definitely prefer the sequel. '28 Weeks Later' is more action orientated and more "thrilling" with a much tighter plot (imo), plus there's none of that boring rapist soldier stuff.
Still love the bit where Cillian Murphy's character goes on the rampage to that amazing music in the mansion. Easily the best musical score in a film for quite a while.
28 Weeks Later's opening 10 minutes is worth the price of the cinema ticket alone. Very intense stuff.