Disclaimer: I own a 706MkIIG, so I'm biased - but I know people who own the 857, and have been able to make some comparisons.
The size is attractive, but the display is tiny - too hard to see necessary information while driving, IMO. Some of my acquaintances complain that the 857 is deaf, particularly on 6m.
There were some reliability issues with the first ones, but those seem to have been worked out over time.
Overall, I get the impression that there's a reason it's less expensive than the Icom. If you need the size, or if there are budgetary constraints, the 857 is probably a fine rig. If you can fit in the Icom, and can afford the higher price tag, you may want to consider it instead.
-=[ Grant ]=-