Ro-WEE-nuh. A friend of mine once met an Owena who pronounced her name o-WEE-nuh, but I'd expect O-en-a for that, instead of Rowena-without-the-R.
I like it very much, especially because it's pretty well nickname-proof. And, unlike many Harry Potter names, it's been steadily in use before, during and after the books, so I can't see why it might be a problem. The Walter Scott association is actually stronger for me, though I'm an HP fan of note, and I'd like to see Rowena and Rebecca as a sibset.
Edited to fix a typo.
This message was edited 4/17/2011, 11:18 PM