The previous version caused problems during iPhoto 4 udpates, the new version runs on 4.03 and 5. I'm staying with iPhoto 4.03 unless Apple fixes iPhoto 5's performance and reliability issues, but I may try this update.
Keyword in iPhoto are odd. On the one hand they work well with SmartAlbums and they're well preserved in Library merges. On the other hand the native UI for managing keywords is weirdly awful. This helps a bit.