A bunch. Unfortunately, I can't play on it properly yet, since it arrived with its left controller being borked and not working in horizontal directions. Replacement joysticks should arrive later today, and I've already gotten my iFixit kit a few hours ago. But my initial impressions are really positive, it's a very capable machine, as long as you don't expect it to run everything at Ultra settings at 900 fps.
The reviews that I saw unilaterally say that the original ROG Ally (not Ally X, which is the second generation that also is on the market) has poor battery life, lasting only about an hour and a bit at full power. Not surprising, since at Turbo it can do up to 25 W of power (30 W if plugged in), and the battery is about 40 Wh. Still, for $400 that I paid for this thing, I couldn't be happier about it. If you have closer to $700-800, though, I'd suggest you look into Ally X. Or you can also put in a modded battery: there are a few guides about putting beefier batteries into the original Ally, which either involve printing a new back plate, or just slightly adjusting the existing one, with different levels of sketch.
Just don't get the non-Extreme chip. There are two right now: AMD Z1 and AMD Z1 Extreme. Just don't get the basic Z1, it's not worth the savings in money.
Do you have decaf by any chance?