Okay, I got the nettop today, finally managed to hook it up and remove some Acer bloatware (tho they don't include too much stuff, I'll give them that).
First of all, a small rant: normally, the 3610 comes with a wireless keyboard and mouse. However, the Polish Acer branch decided to be "funny", and included a USB, wired K&M instead. On top of that, the keyboard has the official Polish layout, which is strange and unintuitive (99.9% keyboards sold in Poland have the English(US) layout, even though it's, technically speaking, illegal, since it's incompatible with the Polish norm and therefore shouldn't be sold - like anyone cares). Stupid bastards.
Apart from that, I'm really happy with it. It's absolutely silent, even under full load (you can hear the fan if you put your ear next to the exhaust). It is very good-looking and very well made - sturdy, good quality plastic, no misalignments of any kind. One drawback is that the warranty sticker is covering the screw which holds the lid, so upgrading RAM or HDD will void the warranty. Stupid bastards again.
As for HD playback - not a problem. On some dated drivers that were pre-installed with the OS (186.xx) the K-On! OP2 file which was tested above played flawlessly in 64-bit MPC-HC. CPU use was around 50% (I'll see how it goes with the latest drivers), stats in MPC: 8 frames dropped out of about 1800 total, jitter 10ms, audio sync kept within -9 to +3ms (which is unnoticeable). I'll post a screenshot later.
EDIT: with 257.21 drivers, jitter dropped to 8ms, number of dropped frames to 4 and CPU use never exceeded 30%.