I just test the newest version of Yahoo Messenger 9.0.0.907 Beta and it works well.
The difference with the initial release of Yahoo Messenger 9 beta which is labeled as version 9.0.0.797 is full of bugs. So I don't recommend the old version.

Yahoo Messenger 9.0 features some lesser known enhancements such as full Unicode compliant to display almost all languages in IMs and status messages, remember status between sessions, elimination of duplicate contacts between Yahoo and Windows Live ID, and ability to bold distinguish online contacts.

Yahoo Messenger blog lists the following improvements been done to the updated version:

1. Viewing your contacts by their Yahoo ID vs. their full name or nickname. Go to Messenger > Preferences > Appearance to set the preferences.
2. New expanded contact list. If you feel this new look is too spacious, go to the Contacts menu and select “Compact List” to reduce the size of the Avatars and condense the height of the list overall.
3. Message formatting in the IM window (the line break after the name rather that starting text on that first line).
4. Fix installation problems for Windows Vista users.
5. Games are missing from the IM window. If you love your Pool or Checkers, uninstall version 9.0 and return to Yahoo Messenger 8.1.
6. The LAUNCHcast plug-in, along with all the other plug-ins like Yahoo 360, still work in 9.0 Just click the Plug-ins button near the bottom of your Messenger window to find them.

Again, download Yahoo Messenger 9.0 from beta download page will only give users a small bootstrapper setup file which will then further download and install complete setup files from Yahoo server inside the small program itself. Users won’t see the full version setup file (although it exists in temporary folder which you have to dig), so it’s impossible to backup, redistribute, or run installation while offline.

So if you need a full version standalone installer for Yahoo Messenger 9.0 beta, here’s the direct download links for all available localized language versions.

US English: ymsgr900_907_us.exe

America

Argentina: ymsgr900_907_ar.exe
Brazil: ymsgr900_907_br.exe
Canada: ymsgr900_907_ca.exe
Canada (Quebec French): ymsgr900_907_cf.exe
Yahoo Telemundo (Latin America Spanish): ymsgr900_907_e1.exe
Mexico : ymsgr900_907_mx.exe

Europe

France: ymsgr900_907_fr.exe
Germany: ymsgr900_907_de.exe
Spain : ymsgr900_907_es.exe
UK & Ireland: ymsgr900_907_uk.exe

Oceanic

Australia & New Zealand: ymsgr900_907_au.exe

Asia

India (Hindi): ymsgr900_907_hi.exe
India (English): ymsgr900_907_in.exe
Indonesia: ymsgr900_907_id.exe
Hong Kong (Chinese): ymsgr900_907_hk.exe
Korea: ymsgr900_907_kr.exe
Malaysia: ymsgr900_907_my.exe
Philippines: ymsgr900_907_ph.exe
Singapore: ymsgr900_907_sg.exe
Thailand: ymsgr900_907_th.exe
Vietnam: ymsgr900_907_vn.exe

Ok, thats all folks Enjoy the new experience of Yahoo Messenger. I'll see you Online.