It's very busy - we cycled it from Loch Ness down to FW in September. It was eye raising in places. Actually found the Inverness to Ullapool road to be much safer & nicer despite the lorries! Definitely a case of make yourself seen and don't let the tourists (which are the usual problem not the locals) bully you. Hi-viz and lights were needed. We found the drivers of bigger vehicles were the best, they knew they could not overtake and where possible we would help them out.
There is an alternative to start off with - though not ideal for narrow road tyres which follows the canal for some of the route up to the Commando memorial, then you have to go back to the road (after a steep climb from the canal). Then you can bale back to the canal again (its part of the great glen way - hard core track) but there is a section and some climbing around a narrow part of the road (after the memorial in your direction) that was not great and I was quite glad to get away from the road.
That's probably the best photo I have that includes the track...
Google the "Great Glen Way" - that is pretty much the only alternative.