I have been using FreeNAS since version 8.01 on a dedicated computer and the software has matured heavily and is the same software used on the FreeNAS Mini. iXsystems primary goal seems to have been data integrity and second is data speed. I think they have nailed both.
Speaking about the hardware, ECC RAM is required for data integrity. Running a RAIDZ or RAIDZ2 (better) is also key to ensure data integrity.
Lets talk cost... Building a comparable system from scratch would be roughly the same cost and at least with the FreeNAS Mini, you know the hardware is compatible. Also this system is targeted at small business or home users that value their data. If you think about it, if you are a small business and you lose some data, it could cost you hundreds or several thousands of dollars, if not more recovering the data. Hopefully there is an automated backup routine established for highly valuable data. Of a business dropping $2000 for a quality NAS product is a good investment.
The review here was fair yet limited as it didn't show off all the features of the FreeNAS software. I'm not sure I would have mentioned deduplication in the same sentence with FreeNAS Mini, the platform is underpowered to do a good implementation of this feature, but that is my opinion.
FreeNAS has a great support forum so if there are any doubts, check us out.