A great piece of kit in terms of it's sound, overall low CPU usage, and value (especially at this price).
There are a couple of points that some people may find annoying:
a) there are no overall gain controls for the Rack, which can make gain staging a bit of a pain. However Slate himself has said that there is a soon to be released Trimmer module to alleviate this issue.
b) the rack window can be a waste of screen estate. If only one module is required then you will find that three empty (default) slots will be visible with no means to hide them. This becomes a big issue on smaller screens where screen estate management is a must.
c) If you want to use an plugin external to the VMR in between two modules (e.g. FG-116 - **native EQ** - Revival) then it could easily become a bit clumsy to manage multiple VMRs on a single channel strip.
d) You need an iLok2
For my own workflow i find it more than satisfactory and a breeze to use!
Update (26 March 2015)
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In response to comment No.1, Slate has now added the trimmer module (which is available as free update).