Spent 2 hours with mine last night - initial impressions are...I'm super impressed. This is definitely better than any multi effects I've had before. Agree with the sentiment about the Obsidian 7000 - definitely usable and definitely gets a lot of the "feel" of the B7K. I'm going to take some time to side by side the real pedal with the sim, but at this stage I think the real one is going to go up for sale.
Octaver tracking using the Bass Octave (legacy effect) is actually really good, in my experience. I can get it solid to low F 95% of the time. Low E is solid about 70-80% of the time, so usable if I'm careful and feed it a strong signal. Did a 30 second test with the simple Pitch as others have suggested - need more time with it to compare but I feel the Bass Octave gets me closer to what I want. Also, despite them being "legacy" effects, they sound MUCH better than my M9. I should side-by-side the two while I still have it.
I think I can improve the octave tracking further by putting a compressor and maybe drives before it...dunno, so much to experiment with! Not yet at the point of putting it on my board but I can tell I'm going to have one hell of a weekend soon running my "real" pedals into it via the effects loop.
I find I work 90% of the time in Signal Flow mode when building patches, I feel like it makes much more sense to how my mind works. Had/having a few issues getting the parallel chains to work as I want them to in my head, can't get the "blend" idea to work but I think that's more my inexperience at this point.
Less cool - looks like mine has a fault. The middle-bottom screen flickers. Not all the time, but I can definitely make it repeatable by doing certain things, such as using Patch Mode and selecting the middle-bottom patch, or by being in Signal Flow mode and adding an activated block. Have lodged a support case with Line 6 and contacted the store I bought it from. They've offered to replace it already though so I think/hope this will be a happy ending. Will keep you all posted.