Yes, $800 is affordable for many divers. It does look like it would be a very useful tool for divers looking to do planned deco. But for a recreational diver who never plans on see a deco screen except in simulations, it looks like so much bling.
For example, I am strictly a recreational diver. I have never planned or done a dive that put me in mandatory deco. I only dive single tanks and occasionally carry a bailout bottle. What would a Petrel do for me that my Oceanic Data Plus won't do besides take an extra $700 out of my diving budget?
if your data plus works, nothing. If you're buying a new comparable computer, that's the Veo 2.0 at $370. With the current exchange rate, the Petrel gets you the following
Digital Compass-if you have to purchase a separate compass, minimum $40
Provides easy data logging- Petrel can upload via bluetooth to your phone or computer to provide full dive logs. The Veo can, but it's a $90 accessory
Nitrox switching-doing a 2 tank dive on a boat, but have different mixes? Petrel has 3 button presses to switch to the new gas, the Veo you have to reprogram between dives for the new gas.
Want full control over your decompression because you're older, at higher risk of DCS, not feeling well that day, etc. You can program the GF's into the Petrel to make your diving more conservative and KNOW what theoretical tissue saturation you are coming up with.
"real price" of the Veo 2.0 is $370+$40+$90=$500. Do you pay an extra $200-$250 for a much nicer screen, more functionality, better resale, use of AA's, for the last computer you'll ever need, from a company that is known for providing over and beyond customer service? You tell me.
For the compass, it is likely easier to remove from your wrist, but that is my main issue with forearm compasses in general. It should be no different than any other wrist compass, but that's why I use a slate when I have to do anything more than check general bearings.