Great visualization tool!!! Connectivity to many different types of data sources. Great flexibility with function/features used to produce the visualizations and make them interactive for end-users.
Room for Improvement:
The Tableau server piece isn't as mature but it getting better with time. I think they are missing a Power User setting in the administration/monitoring tools that will allow people to see what's going on with the server, extract refreshes, etc...without having full admin access.It's expensive to use at an enterprise level. Performance can be tricky on large datasets (>10 or 20M records). Tableau can handle the large volume, but what/how you use the features in the viz can have significant impact on how snappy the viz is for end-users.