As title really, I have driven past parts of it and certainly a lot of the paths look reasonable but I wondered what the rest of it was like?
As others have said essentially almost all of it is rideable. There is a long section at the top of Loch Lomond which is more of a scramble than a hike so some people prefer to jump across the loch on a wee “ferry” at Inversnaid and then follow the road to the Drovers inn and rejoin the path their. Others think its not so bad as to justify several miles on busy tarmac. That decision will depend on your party’s skills and fitness levels; the bikes you have and how much gear you carry.
As others have said its not necessarily the best MTBing in the “area” – but if you fancy the idea of a “journey” and don’t want to spend hours and hours trawling over maps trying to find three days of routes which all join up without lots of road in between then it is perfectly acceptable.
I’ve walked or ridden all of it – but never as a single trip. Its certainly better as a bike ride than a walk as you can “munch up” the boring miles.
my girlfriends dead set on us doing this next year, would it be fine wild camping all the way up/down?
In principle you are fine (beware new restrictions on Wild Camping on E. side of Loch Lomond come/coming into force – although probably not affecting anywhere you might want to stay.
In reality you don’t need to camp, there is cheap hostel / bunk house type accommodation (as well as some better nicer stuff) which will be much more pleasant if it is bucketing with rain or swarming with midges (or both) – which is a distinct possibility. Obviously at peak times this will need booked well ahead. This will save lugging not only a tent but also sleeping bags, towels, etc which unless you have a Bob trailer is probably a major PITA on a trip like that.
Its certainly been discussed here and on many other sites extensively. Ignore anything pre 2003 which says cycling not permitted, but bear in mind it was intended as a long distance walk and not all walkers are as welcoming as you would hope – although I’ve never had a problem if you are polite and ride responsibly.