OK here is my feedback.

First some superficial stuff: Installshield is not going to fly with the market you want to make real money off, i.e. big organisations who want to deploy through Active Directory. The Office addin installer can be put as a merge module into an MSI, that will make the overall install experience much slicker. The instructions on the web page that opens after installation say to look in tab 'Add ins', but it's in separate 'Slate' tab (Office 2013/Win8.1). The introduction tutorial is unintelligibly small on a high dpi screen (Lenovo Yoga Pro 2); so is everything else that doesn't scale with ctrl-mousewheel. I just aimed very well with the mouse to make it go away and I tried to figure it out on my own.

On to the functionality. I work with Excel models if not daily, then at least a few times a week. These are usually either models I get from partners and which I integrate with other models, or they are data analyses. Over the last few weeks I worked with an economic model and a demographic model I got from outside, and I did several analyses on land use/population data myself. They're always intricate and on the overall complexity scale (as far as Excel models go) range from 'medium' to 'high'. So I think I'm exactly your target customer.

I tried first with a medium-complexity spreadsheet (5 worksheets, couple of dozen 'variables' where the variables are usually 2d matrices and the analysis is in aggregating and disaggregating the input data in various ways). I tried Slate on several fields, but for none of them I could better understand what they were than by using Excel's build-in 'Trace precedents' or even by just reading the formulas. Slate shows only very little context; you have to scroll out far to see those 2 or 3 steps away, but at that point everything is too small to read. Furthermore, it doesn't really dissect formulas; for example, an 'if' with two VLOOKUPs is just a pink blob with a bunch of arrows coming in, without giving information on how or what. It also takes up half of the width of my screen, so there isn't a whole lot more I can see on the screen.

What is worse is that there is no description of each 'block' (could have taken the label in front or above the formulas in the original sheet); nor does there seem to be a way for me to enter them manually. For example, I can see the value in being able to annotate a 'box' (variable) in my model with 'Plot counts per district', then an out arrow with 'multiply with average amount of dwellings in low residential land uses for the transport zone', then the box where the arrow ends up with 'dwellings in low residential housing areas per district'. That doesn't seem possible.

When 'show detail' is on, some of the boxes have the upper left corner of some range of cells in them; I'm not sure how or why. They just seem to take up a lot of space.

It doesn't look like I can edit anything in the result. For example, I have a lot of 'if(iserror(formula), 0, formula)'. I don't want to see all of that - I just want to see 'formula'. The rest is an implementation detail.

When 'hide detail' is on, there doesn't seem to be a way to jump back to the Excel sheet.

I had a more complex sheet I wanted to try it on after this one, but even this medium complexity one was harder to understand (and I wrote it over the last few days, so it's still all in my mind) than the original Excel sheet is; so I didn't bother with the more complex one.

Sorry to rain on your parade, it must have been a lot of work. I don't see much added value though in its current form. I'm sure that with much polishing it can become useful. I think annotations should be first; then abstracting away implementation details of formulas, you have more ideas yourself probably. Slate also loses the spatial relation between e.g. columns next to each other; I'm not sure what to do about that, but it should make up for that somehow.

Lastly, the price is ridiculous at this stage. At first I though 'oh 50$ seems OK, then I saw '/month/user'. $50 one-off seems OK; I don't see who will pay 500 a year for this, especially with the basic functionality it has now.

Feel free to email me if you want to know more about the spreadsheet I tested it on. I'd love to be shown what I should look at that would give me more insight in my model; I didn't see it myself. A real-world example on your website (as opposed to an Iron Man made up one, which to be honest is quite juvenile) would maybe help, too.

Good luck with your product. Please don't be discouraged by this - I'm not trying to put you down, just honestly telling you what my experiences were, as you'll get very few reactions from people like me who decide to give up after 10 minutes; while those are exactly the ones you want to hear from to know what to work on.