Directory Service as Meta Directory

Hello Everybody

I’m studying for the MCSE and I had discussions with a few peoples about Meta Directories. Now I would post the question in this Forum which Directory Service do you recommend as a Meta Directory, and why.
Is the MS AD really notrecommendable as a Meta Directory?
Why?
Which one does a good Job?

Thanks in advance for any answers.

Greetings
Salvatore Cagliari