Lots of folks on here gave me info when I was planning this so I thought I would post my impressions for anyone else thinking about it.
We did it clockwise. My feeling is that whilst anti would still be good clockwise is the best way for rideability both up and down.
The best bit of advice I got was to look for any easier alternatives on the climbs like landy tracks. We didn’t do this on day one and ended up with a 2 hr carry up from Trelechamp.
We started at Trelechamp and carried up the Aiguillette des posettes which was about 2hrs. The descent to the col of the same name was good but short and extremely technical. Given the carry it wasn’t really worth it. We contoured round to the Col de Balme which gave an awesome and not too hard (red) descent to Trient. At the top of the col are TMB VTT signs which are best ignored.
One point worth noting is that the recommended map IGN Pays du Mont-Blanc does not show detail like buildings in Switzerland and Italy. Also Trient is not marked at all and there is no distinction between double and single track.
The section from the Col de la Forclaz to Bovine looks to have been recently re-done and is mostly rideable (up). From Bovine round to La Jure is a f***ing pig what ever direction you do it. I couldn’t see any alternative to this that didn’t involve a massive detour on the road.
La Fouly to the Grand col Ferret would be mostly rideable in the dry. We had a rotten day and the lack of traction defeated us. As the weather was so poor (snow to 2000m) we missed out the Bonatti refuge / mont de la saxe section and rode down the road instead.
Having been told the climb out of Courmayeur on the walking path was a carry we rode up Val Veny instead and picked up the TMB at Lac de Combal. Most of the way up the Col de la Seigne is rideable. More if it was dry!
Les Chapieux to the Col de la Croix du Bonhomme is a slog but again more rideable in the dry. The descent from here is one of the best I have ever done with only a couple of lifts over unrideable obstacles near the start. This would be a poor choice on the way up and most of the routes I saw online avoided it if going anticlockwise.
For the rest of the way to les Houches we followed the walking route to the letter and it was fine.
Overall thoughts? Like everything else the weather will make or break it. We had 1 nice day one cold and showery day and 2 days that just pissed it down (snow on the passes) from start to finish. This lead to a lot more carrying than might otherwise have been the case and lead us to the conclusion that it was only just worth it. Given dry trails there is no doubt I would have been much more enthusiastic. Clothing wise I would say (forecast depending) you should take what you would for a remote ride in Scotland in the winter. I made a schoolboy error on the clothing front one day and ended up border line hypothermic.