I agree about avoiding the S5110b for what you need. The 5110b's dpad is VERY inaccurate and tends to slide around, I had to mod mine to make it work accurately, and the battery life sucks unless you put in a new battery, wifi usually is terrible, it's dual core, and the screen is pretty poor.
Overall the S5110b is a lot of hassle for a dated system that you'll spend a lot of time and money modding to make it acceptable.
I have a G7 and I modded the dpad by removing it from the shell entirely and sanding down the "top" of each of the 4 cardinal directions. I did not like how the ends of the dpad stuck up so far and were "sharp" feeling, especially when sliding between diagonals or doing hadouken-style moves.
If you want to try a similar mod here's what you do:
Disassemble the device and remove the plastic dpad.
Get some sandpaper that is around 80 or 100 grit, lay it flat on a table.
Place the dpad piece upside down on the sandpaper (so that the part your thumb touches is what is being sanded) and sand it down while keeping the sandpaper stationary.
This should equally sand down all four directions at once to keep them the same height, just remember to take it slow and apply equal pressure to the dpad, don't press down on one part as you sand, or you'll end up wearing one direction down more than others and you'll have to compensate. Stop every once in a while to check your work and make sure it's progressing well, and check if you think it feels good or needs more sanding.
Once you've sanded it down to an adequate height, you can optionally get some fine grit (~600 I think is what I used) and sand it some more. This will not remove almost any plastic, but will "polish" the parts you sanded to make them smoother and more fine under your thumb. You can even finish with some 1000 or 1500 grit after this to make your polish even more smooth, but you're getting pretty nitpicky at that point.
I found that removing these sharp upward curves at the edges of the dpad made it TONS more comfortable to use, I cannot fathom who at GPD actually thought those sharp upward points were a good idea for using longer than even 10 minutes at a time...I'm convinced that nobody actually tests these devices for comfort prior to shipment if my experience with them is any indication. Or maybe Chinese hands find these comfortable while my long american fingers don't...I'm not trying to be rude, I just cannot imagine anyone finding this dpad comfortable or usable for something like a fighting game, which is a MAJOR sector of the retro-game space (and ironically enough, is always in the demo videos that companies like GPD send out).
I can't speak about the cardboard mod that skelton mentions in his post - on my G7 I tried putting plastic under the dpad to improve it's precision, but it didn't seem to make much improvement, though I used pretty thin plastic, so maybe thicker would work better. My biggest problem with the dpad was its precision - I would often get left/right presses on the dpad when attempting to just press straight down, so my assumption was that a thick backing behind the dpad would make this problem WORSE, not better. Maybe I should try it just to be sure.
My reference game(s) to test dpad/buttons with? Ninja Gaiden 1, 2, or 3. If a dpad can stand up to the precise demands of the most hardcore action game I know, it's worth using. The G7 did not pass this test, even after my current mods
But it did get a lot better vs the stock dpad, which was painful after even 5 minutes of hardcore ninja action.
