hp 50G build quality in today's standards (2018)

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The coating of the cursor keys on my 50g (built 2010, week 38, according to the serial number: CNA038...) is starting to wear off. Other than that, it's in good condition on the outside, and the keys work like a charm (though I wish they were a just little less stiff - the Casios' rubber sheet is horrible; a rubber dome would be nice according to my experience with PC keyboards, but it probably wouldn't fit inside a calculator case).
By the way, (not really relevant to the topic, but) I have a 1GB SD card in black that's pretty much permanently inserted. With the 50g's black case, it blends in really well.

On the inside, the situation is less pretty. As mentioned elsewhere on this forum, it suffered from the high-current-draw-when-off problem, which appears to be caused by a leaky capacitor. The capacitor has been replaced, but the repair job apparently wasn't perfect. It's been subject to spontaneous shutdowns ever since - and every time it happens, it reboots when turned back on the next time, just like after a power loss. Luckily, USB power seems to make it happen much less.
The shell is difficult to open (mine has a few marks along the edges as a result), and the aluminium sheet or whatever this thing is wrapped around the board tends to be in the way (is this supposed to be some shielding? Haven't seen this in other calculators so far). Other than that, it looks about as sturdy and serviceable as any modern board with SMD parts on it.

I can't comment much on older HP calculators. There is a disassembled 48G lying on a nearby shelf, but it's been in that state for at least 15 years. The story behind this device is that the original owner, some random student, had it with him in a car accident. It fell into the water, the insurance got him a replacement, and this one was given to my father due to his reputation for being able to fix electronics (sometimes by merely being present). The student hoped to have an extra operational 48G for cheap, but this time my father failed, and the device ended up in a dark corner of my family's house until I found and recognized the faceplate and asked my father if there are other parts of this calculator somewhere. I did manage to boot it up once, but I gave up as well in the end. I can test the feel of the keyboard, and it's about as stiff as the 50g's. The board was mounted inside the shell in a horrible way though, those metal bits seem like they are designed to break while servicing, and the keyboard and display connections look like they are prone to misalignment and poor contact. The shell isn't easy to open either, and even harder to close seamlessly because the faceplate is probably bent and curled up afterwards.

I have opened a Casio Algebra FX too. It's one of those samples with a failing flash (or maybe a few failing solder joints nearby), and as I've written above I'm not a fan of the rubber sheet keyboard (I have a hard time feeling when I've pushed a key down far enough to activate it). Right now it's lying next to the 48G, and disassembled as well. It's fairly simple to open though, just four big screws in the top section of the back half plus four smaller screws inside the battery compartment located in the bottom section. With those screws removed, the back half can simply be lifted off. My brother opened his 9860G once for some hardware modding, and it was a similar experience. My 9750G+ has screws in the same places as the AFX, but I didn't have a reason to try opening it yet. According to reports on the internet, the back half of this calculator series is additionally held in place by some sort of clips. They all have the same rubber sheet keyboard feel.