Posting 3 months after the original question, I know that the OP will not see this response, but for others searching this forum I will share my opinion. I have the GDHE 50 which is very similar to the GPHE except that it has both direct exhaust and intake vents and increases the btuh from 76,000 to 100,000. I had it installed about 2 years ago. At a little over $2,000 (which included installation/disposal costs) it is much more money than a conventional tank WH and will take me almost 10 years to get to the breakeven point from gas savings. Mine did qualify for the federal tax credit (and gas company rebates), which took some of the sting out of the price. For installation, I needed to up size about 4 feet of gas line in addition to adding the PVC intake and exhaust pipes. Although it only needed 2” dia. Pipes, I used 3” in case I ever wanted to replace it with a 200kBTUH tankless. Other than that, it installed just like a 50 gal. Tank WH.
The good: very efficient (2/3 gas usage over previous WH); FAST recovery (takes less than 15 minutes to get to 120F from 55F when returning from vacation), 50gal Vertex will replace a 75gal conventional.
Not-so-good: noisy when actively heating. About as loud as a refrigerator, but more like a jet engine starting noise (fan noise from exhaust fan). Required up-sizing the gas feed line to the Vertex. Moving parts implies more repairs.
The bad: It stopped working once (at 20 months) when the exhaust got clogged. The inside of the heat exchanger shed enough rust flakes into the U joint of the exhaust and clogged the condensate drain line. The unit's computer diagnostics were helpful in pointing to where the problem was; something like “Error – blocked exhaust”. I cleaned out the rust and it's been working fine since then. I hope that rust blockage doesn't become a regular maintenance item, otherwise I'll be testing out that AO Smith warranty.
To those critical of my Vertex choice, please know that my machine room space and gas line infrastructure would not support a tankless or a 75 gal conventional WH. With the original 50 gal WH I had, I was always running out of hot water. I'm very satisfied with the performance. Reliability? - um, we'll see.