The app has gotten fixes to make sending and receiving MMS messages more reliable. It also has a new settings toggle to help with an annoyance where Hangouts would convert group messages to MMS, which could cause some users to be charged by their carriers. Now, you can turn that behavior off if you are one of those unlucky users.
For the really advanced users, there is also now an option to manually configure your APN settings. Hangouts had automatically used its own APN settings when sending MMS messages, so if you had custom APN settings, you may have had problems with MMS messaging. This new option should help you sort that out.
The update is rolling out in Google Play right now, but for the less patient of you, there are download links at the source.
source: Android Police