I am using Omni 8.1 (Build 20171231), no problem, stable enough for daily use. Flashed it with unofficial gapps nano and openbeta29 modem+ firmware package.
Clean flash is recommended, I tried dirty flash from Omni 7 Build 20171129(around this time) and found there were so many weird bugs that shouldn't be there from the first place.
At first, the most difficult thing is to find the unofficial nano gapps for Oreo 8.1 that also must be compatible with Omni 8.1.
Then, there are some applications (on my case, drastic and Firefox) reported some libraries can't be found. "Detected problem with app native libraries ...". It is still running normally though. Firefox crashed sometimes when being used heavily (browsing and switching between windows, maybe just Firefox thing).
PicoTTS kept crashing no matter what I did, so I used Google TTS instead and then remove PicoTTS from TWRP File Manager.
While it feels smoother in Oreo (Omni 8), combining Omni 8.1 with AKT profile (Project Zhana's) did help me to squeeze longer battery life. Omni 8 seemed like it doesn't have init.d support so I just run the profile script manually every boot. (Might be applicable via Tasker? I dunno)
Google camera HDR+ port is running normally and I can use potrait mode (motion photo is disabled as I don't use Google photos, I didn't see any difference between motion photos and normal photos). Back HDR+ is working while front HDR+ and also front potrait mode are broken.
Also, on Omni 8, the customizations are quite limited compared to Omni 7. I am still okay with it though.
Other than that, everything is quite similar.