I have been using SOBI2 for a few years and although I'm not a hardcore developer, there was so much support from the community and the developers for this that I was able to create a great directory site!
SOBIPro takes that a step further with even more enhancements....things like the built in repository for plugin updates, multiple directory capabilities and great documentation make this a very worthy successor to SOBI2! I look forward to even more great applications from Sigsiu!
To respond to the review that said SOBIPro couldn't handle large directories: I have over 1300 entries in mine using SP v1.05 and I have no issues with speed on my site. Not only has support been terrific, but the 3rd party has been working quickly to support SP like it does for S2.