When navigating downloaded GPX tracks it will sometimes take a diagonal shortcut between two arbitrary track points, skipping over roads. This is my main issue and it's more prevalent over longer more complicated tracks. Some issues with auto-stop-start function. Errors with distance when used with a powertap hub. These things depend on the version of f/w. Version 3.3 came out a month or two ago and a some have tried it and gone back to 3.1. Version 3.2 came out ~6 months ago as was pulled because it was too buggy. I've had equal luck with 3.1 and 3.3.
If I were to buy right now and had to decide between the two, I'd probably to for the 800. It's got bigger problems than the 705 but I'm sure most will get ironed out over the next 6mo to a year.