Never owned a 5th gen but have rented and driven extensively in several of them across most trim levels. Comparing the two, the only major downsides I see to the 6th gen are the less than smooth transmission and the crappy backseat folding -- but as others have said, the 5th gen was sluggish, 5 speed, and was known to hunt gears and hesitate as well, and as I pointed out, the 5th gen rear seats didn't exactly lay flat for sleeping either. Other gripes like a plasticky interior or cabin noise -- these things could definitely be better, but I still don't think it's a downgrade compared to the 5th gen. Just more of the same, if anything.
I'm 6'3" and don't feel like the 6th gen is particularly cramped? The seats are definitely more comfortable and more adjustable. I did specifically shop for one without a moonroof because I knew headspace would be an issue, but that's an issue in lots of vehicles for me so I've never had one and don't miss it. I also have zero issues getting in or out even with the moving seat turned off, and never would have even thought about the lack of a grab handle on the pillar if people didn't mention it. The only thing I dislike about entry/exit is the damn running boards that stick out too far and always get dirt/water on the back of my pants. Annoying, but fixable. Planning to replace those with lower profile sliders soon.
Overall, I'm not arguing that the 6th gen is perfect, but there are very few areas where I feel like the 5th gen was superior (longevity/reliability aside, which only time will tell). At worst, most of the little gripes are just classic Toyota, not new or unique to the 6th gen.
Have had an 04 SR5, 18 ORP, and 21 ORP and now a 25 ORP hybrid. It pulls my trailer with SxS at ~4000lb like a champ. The only thing that I would change is go back to how the back seat folded like before. Other than that this one is what all the others should have been. Very happy with my 25