Underground was good, but they should have stopped the tuner car stuff after NFSU2.
I am however, EXTREMELY excited for the new NFS: Hot Pursuit that Criterion, makers of Burnout, are working on.
I've been a NFS fan for a while... I have:
NFS2
NFS3: Hot Pursuit
NFS: Hot Pursuit 2
NFSU
NFSU2
NFS: Most Wanted