Whitlow, just noticed your comment about why you posted about this toilet. Absolutely!! It's always interesting to see what is out there. Delta is a respected name, and they seem to have an interesting marketing approach. Glad you shared your experience.
One of the big objections raised on here to brands like Glacier Bay and the Costco toilets is that finding parts is a pain and models change as fast as the next shipping container rolls in.
Upon further review, it appears that Delta has made every critical part to these toilets available at HD [Online Only], and the parts are not outrageously-priced (Fill Valve $24, Flapper $12). Knowing Delta, they likely have these parts available directly from the factory as well. At present, it looks like the flapper for both toilets and the special fill valve for the Corrente are out of stock online, but I assume that will change. The fill valve for the Prelude should be replaceable with the Korky 528MP and others. That said, it would truly suck if one's flapper failed and HD, the only retail source, continued to be out of stock online, as there likely will be no generic replacement made for this flapper, as it is a very odd size, unless Delta achieves truly-huge market penetration.
Delta's marketing plan to differentiate this toilet is flat-out "ease of installation, including ease of removal of old toilet". It's intended to be DIY-friendly. I assume that the reason they don't use tank-to-bowl bolts that penetrate the floor of the tank is based upon some research that shows that low-experience DIYs are challenged with leaks at the bolts and (as we see often on here) difficulty getting the tank seated properly.
I commend Delta for creating some very-well-produced, slick, thorough online videos regarding both removal of your old toilet and installation of your new one. Unfortunately, I'm not sure I agree with a number of their instructed installation steps, particularly (1) trying to stick the wax ring to the toilet rather than putting it on the floor, (2) putting a bead of silicone on the base before installing it, (3) using silicone instead of polyseamseal, (4) no mention of dry-fitting the toilet base first, and (5) using a wax ring with funnel. Of course, almost all of these are preferences derived from Terry's recommendations but the logic of Terry's way is really indisputable. I also don't recall anything in the video about ensuring proper flange height to go with the standard-thickness wax ring they give you.
Given the price point, and the fact that they include everything in the box, including the equipment to remove the old toilet, a pair of gloves, a sponge to remove the water in the old toilet, etc, I would be curious as to how it's built and how it performs.
Whitlow, the overflow riser is the hard plastic tube that rises from the flush valve to about 1/2" above the expected water level in the toilet. It is designed to bypass the flapper. It has two purposes: First, if the fill valve fails and keeps running, the tank won't overflow onto the floor; rather the water will run down the overflow riser, past the flapper, and through the bowl to the drain. Second, it receives the water from the refill tube off the fill valve and refills the bowl water spot while the tank is refilling. Recently, it appears that more manufacturers are obstructing or narrowing the opening to this riser, which would appear to reduce its ability to carry off any overflow from a runaway valve.