I have tried it ~1 month ago and was quite a bit underwhelmed - direct connectivity with Google products is nice, but the data manipulation capabilities themselves lag behind Tableau a big deal (especially when it comes to clicking around in charts and tables to slice your dataset, which is admittedly something in which Tableau excels).
I do believe it is a valuable addition to the GA enterprise tier (especially for customers savvy enough to use BigQuery too), but at the moment I don't quite see it as a serious competitor to other off-the-shelf BI tools - would be very happy to be proved wrong in a few months, though.
