Strigoi are, without a doubt, the most brutally effective in
HTH of all the vampires. And they don't lose power dice the way Blood Dragons do. They also give you a fluff excuse to field tons of cheapo ghoul units, which can flee and give amazing tactical utility to a
VC army.
Of course, all of this will likely change with the new book, especially with Ghouls going to being ranked undead.
If you're looking for some info on their current (old) capabilities, there's a guy down in Australia who's written some excellent tourney reports:
http://warhammer.org.uk/PhP/viewtopic.php?t=15235&highlight=
http://warhammer.org.uk/PhP/viewtopic.php?t=16461&highlight=
http://warhammer.org.uk/PhP/viewtopic.php?t=25494&highlight=