What is our primary use case?
Since we are a consulting company, our cases vary from customer to customer and from department to department, but mainly we have found two "standard" cases: Business users connecting to a DWH, or trying to consolidate several Excel workbooks to create business dashboards.
In the DWH cases, it is simpler since most of the data transformations and modeling are done by IT, but in the 'spreadsheet world', it becomes more complicated.
Users try to move from a huge table filled with numbers and "color-coded" cells to data visualizations, sometimes not even knowing how to write a simple Excel expression.
How has it helped my organization?
The biggest improvement comes from a "do it once, refresh many" concept. The users used to perform a lot of massaging on the data itself prior to the analysis phase. Now, they build their reports and refresh the data, so they have a lot more time to perform analysis, which is the real added value to the business.
Unlike other tools (like Tableau), getting to this point took a lot of resources in training, but when the users got there, it is a real time saver for them, allowing them to focus more on analysis, not on data manipulation.
What is most valuable?
I like the downloadable visualizations a lot. I think that they give the user a very nice starting point in creating awesome dashboards, with the right way to tell their data story.
Data Modeling is very simple and powerful for an experienced user, but I think it needs some improvement for the non-technical people. Besides this, it allows us to save a lot of time in data "massaging" prior to the analysis phase.
Reports are quite easy to build, even when some programming concepts are needed. Some users struggle with DAX a lot!
What needs improvement?
I think Power BI needs to focus more on the end-user (i.e. the business user). Solving things in DAX is not a nice approach to a user that has no knowledge of IT nor wants to get it.
Some kind of expression wizard, where the user can build a complex expression without any knowledge of DAX, can be a good addition to the product.
I see a lot of materials on how to go about doing things in Power BI, but Microsoft can add a "learning path". It would instruct users where to start, then what to learn next, etc, to sort out their learning journey.
For how long have I used the solution?
I have been using Microsoft BI for two years.
Disclosure: I am a real user, and this review is based on my own experience and opinions.