We just picked up our Grand Design Reflection, so we certainly can't speak to the long term quality of the unit. We chose this unit due to the floor plan and based on excellent customer experiences we read online for Grand Design (including the GD owner's forum that's reference earlier). Right off the bat, I found a few issues, but nothing notable. I'm the type that will spend a few dollars and time to fix an issue long before I drag it to the dealer for warranty repair, so the couple issues I found are of no real consequence.
We bought this unit from a large RV dealer that carries more brands and models that you could even imagine exists. However, when I was having a totally offline conversation with a service tech at this dealer, just trying to pick his brain, he said that Grand Design has the lowest number of warranty claims in their shop than any of the other brands. In fact, he was in the process of procuring a new GD Solitude for himself. In his words, the ratio is like 1 to 15 (GD vs. Forest River brands) in terms of what he sees come through the shop.
For the record, Montana (and others) do not have a floor plan that suited us. We started by finding the floor plan we wanted, then tried to select the best manufacturer. We had it narrowed down between GD and Jayco, and we're happy with our choice.