Hi,
I've recently been in the same position as you - found a store demo of a custom bike in my size, but in my case, I couldn't test ride it, because it was on eBay. But since I've extensively researched the geometry that I need, I was willing to go for it.

My experience with it so far - it's definitely the right size, but I am going to be tweaking with it for a while, so I certainly didn't have a "Eureka" first ride. My store demo appears to have been put together with a few random leftover-type parts, so one of them was really wrong - crankset way too big for me. This has definitely affected my experience with the bike so far, and I am working to fix this ASAP.

Even for parts that I know are appropriate, like the WSD-style handlebars: they are a new style of handlebar for me, so I've been tweaking the angle a lot, trying to get the best hand positions.

To summarize: Try to observe if the various parts on the bike (that you're not going to swap for your test ride) are ones that would make sense for you. Those could affect how you feel on your test ride, but can be fixed later if the frame is a good fit. Ditto for parts that are significantly different from your old bike in some way - it may take some time to figure out how they work for you.

Have fun - and if you're thinking of buying one of these demos, good luck with it, b/c opportunities like that don't come around very often!