It's a great idea, concept, and I'm glad it's there. Bravo. And with the link posted in the other thread, it was remarkably easy to get one going.
That said...
Note: I do know the answer to many of these having read the documentation after my initial dive-in. I just wanted to see how easy the use of the data grid was. I still believe - even after going through the docs - that many of these still are issues.
It took me all of 10 minutes playing around with it to hard lock up Rev. Admittedly there will be bugs, I'm sure. But even using "top" and "kill" from terminal in OS X failed to kill Rev. Note: this was caused by modifying the basic template and following the tutorial to fill the dg with data. However, I forgot to modify my layoutControl handler, which didn't find the controls that it defaulted with. Bad times.

Buttons I place in my row template can't be debugged. Perhaps this can be worked around by just making them call commands on another stack somewhere, but was a serious bummer - especially while still learning how to use it and didn't know if I was doing things right.
It was disappointing that I couldn't reference the other controls in the row template group by name and have it "just work." For example, if I added a button in the template that would answer the text of a field in the template, I couldn't just do:
answer the text of field "foo"
Well, I could, but it always returned field "foo" of row 1, instead of the group row the script executing was on. Instead, I needed to access the actual entry from the dgData. That's fine for the control properties that get set by dgData, but what about the ones that don't? What if I wanted to change the background color of the field when I clicked the button?
If I modify the row template (even just a script in the template), I need to click "Refresh Data Grid" to get those changes to take effect. This took me a while to figure out on my own. Not a big deal, but definitely an extra step that was a minor annoyance.
I found it sad that the geometry manager doesn't work [well] for the row templates. I'm sure this isn't an easy problem to solve, and is why so much control was put in the hands of the programmer with layoutControl. Still, it would have made life so much easier if it did.
Aesthetically, I think it's quite sad that the dg control doesn't follow any of the regular Rev property conventions. Properties are named with spaces in them (like "allow editing" and "alternate row colors" as opposed to "allowEditing" and "alternateRowColors"). This means I can't use the get command on the dg like I can every other control. Or, if I can, it isn't so obvious how. I later discovered these are accessed through dgProp custom property. Still...
Similarly, do the dg properties begin with "dg." If that was the convention throughout Rev, that'd be cool (and I prefer it in my C code). But that isn't the Rev convention. So it really sticks out with a sore thumb. I don't do:
set the fieldText of field "foo"...
So why should I be doing:
set the dgData of group "foo"...
instead of:
set the data of group "foo" ...
I'm sure there are reasons behind it. And I've only been playing with the control for an hour or two, so hopefully someone can just enlighten me.

Here's hoping the data grid evolves into something wonderful!
Jeff M.