The free version of the service is too limiting to be very interesting to us. However, it is available from Microsoft resellers as a standalone for about $10.00 per month per user with no contract.
The free desktop version is not limited. It's very powerful. The growth rate is astounding. We get significant new features every month, sometimes weekly.
If you compared PowerBI to Tableau 6 months ago (or even 3 months ago) you are out of date. Its data model capability is far superior to Tableau. The formatting and graphics capability is catching up quickly. The intuitive UX is first rate.
https://powerbi.microsoft.com/en-us/pricing/