What are the benefits of Views 3? I am exploring a reason to upgrade from Views 2x to 3x.

What major benefits Views 3x provides?
Are there any major API changes?
There are hundreds of improvements. There are also many API changes, but they would only affect custom code using Views hooks, not the views themselves, which are converted upon update.

Some major features: 


Functionality similar to Semantic Views.
Group-by support and aggregate functions: you can do queries like "give me the count of nodes written by each user, sort by the amount of nodes DESC."
Views OR: you can separate different filter conditions by OR.
Pluggable pagers.
Pluggable exposed filters: expose items per page, sort.
Header, Footer, and Empty Text as pluggable areas.
Pluggable caching.
More argument, filter, and sort handlers.
Pluggable backends Flickr API, Apache Solr, RDF, etc.
Displays can now be disabled, renamed, and reordered, cloned.
A new UI in the Drupal 7 version.


Most importantly, it provides a much smoother upgrade to Views 7.x-3.x.

See the CHANGELOG.txt for a complete list of changes.