I've done all of it except Ilfracombe to Barnstaple (one of many targets for this year).....strictly speaking I haven't done the bit from Tavistock to Yelverton either....I always go on the A386 there as I'm not a mountain biker.
Barnstaple to Bideford is dead flat, tarmac and open with good views of the Taw and then Torridge estuaries. There are a couple of signposted wildlife habitats on route. At Fremington they have reconstucted the station and it's now a cafe and bike hire place. There is a restored signal box at Instow if you are in to that kind of thing. At Bideford there is another restored station and cafe, the town is well worth a look round.
South of Bideford the Tarka Trail heads for the trees whilst still following and going over the Torridge a couple more times...there is also a well lit tunnel to ride through. The station at Torrington has been converted in to a pub with a beer garden. After that the trail, leaves the Torridge and follows a stream, again through woodland, the surface is still firm and sealed as far as Watergate Bridge....after that it's a little rougher but still rideable on 23s but you do have to look where you are riding.....you can also feel you are going slightly, but constantly up hill. There is another small cafe at East Yarde. It's downhill after that and again it's quite rough in places with ballast sized rocks dotted along the trail...again still rideable but the next time I ride it (I do it about 4-5 time a year) I'll take the country lane from East Yard to the entrance of the clay works. From there it's smoother though not tarmac....more trees and then a diversion off the old railway for a while (easy to follow) before you end up at Meeth.
You then have to follow the A386 for a while (reasonably quiet) and you arrive at Hatherleigh which is worth a quick look round. The NCN then takes you on a minor road to Jacobstowe which I have done once but it is hillier than the A386 and then B road route which is perfectly quiet and quicker.
Jacobstowe to Okehampton is a constant drag uphill on a minor road. The NCN makes you climb further uphill than you really need, I see there is a picnic site in the woods it passes through but I always tun left at the crossroads just before the woods and save about 35m of climbing....either way you pick up a B road which takes you in to Okehampton. Through traffic is signposted to turn left as you go past the Heinz depot but carry straight on to reach the high street and save more climbing.
The NCN route out of Okehampton takes you uphill to Okehampton station where you join The Granite Way....the highlight of which is the Meldon Viaduct...there is a cafe beside it in a converted rail carriage. This section of trail carries on the old railway to Lake viaduct where a bumpy path takes you to the main road (or you can actually keep going staight on and after a slight footpath diversion pic up the main road a little further down) though a diversion at the chuch to the Highwayman pub at Sourton is thoroughly recommended....it is unlikely that you have been in anything like it before. The NCN currently goes on a bit of a needlessly hilly route to Lydford all to avoid a bit of A386 which isn't that busy and also has the added bonus of another good pub, The Fox & Hounds and a tunnel of trees down to a recently opned section of converted rail path. There is a plan to link up the two sections of old railway using a path that will have to go near but not through Bridestowe station, a private residence now.
Lyford is quite interesting with the castle beside the road and also Lydford Gorge (National Trust) which you can see bits of from the road which goes over it. South of Lydford you have a choice of NCN routes. You can head southeast and pass through Mary Tavy and Peter Tavy but that route bypasses Tavistock....you'd also miss Brent Tor with it's tiny church balancing on the top of the rock....not to be missed....and neither is Tavistock.
From Tavistock it's country lanes for a while but then off-road and I hear it's rough but I've not done it myself. There was some excitement this time last year when the eagerly anticipated converted railway route was being prepared but they've announced it will be remaining suitable only for mountain bikes for the time being and it's also rather disjointed at both ends.
The section from Yelverton to Clearbrook/Goodameavy I'm not familiar with either as again I take the main road and then minor road before picking up the Plym Valley Trail at Goodameavy which is tarmac all the way to Plymouth via a bat friendly lit tunnel, several viaducts lots of woods, a spot for seeing peregrine falcons from the most southerly viaduct (a bird group installed cctv on the cliff face beside it's haunt and you can see footage of it on a tv in a shelter and look through telescopes on the viaduct).
The last section of the NCN goes beside the Plym estuary, the Saltram House estate (well worth a stop) and the local landfill site.
Some pictures to follow tomorrow.