I've been taking this line fairly often recently, and I can see how it has improved slightly over the years since when I used to take it years ago. I've generally been getting it at Umberleigh (as it's the closest station to where I'm currently living, and I can park there for free!) and being able to know that I can get an Exeter train at ten to each hour (pretty much) is useful, and the last time I took it it was rather busy - I caught the 7.51 from Umberleigh and had to sit on one of the pull-down seats by the doors (was a 150) - although from the guard's comments it seems fairly erratic as he said he was on the same service the day before and it was much emptier.
I've also been surprised at how many people are using the intermediate stations, usually Morchard Road and Copplestone seem to be almost guaranteed stops, often Yeoford as well.
My reason for commenting here though is in reply to the comments about Newton St Cyres and Chapelton - I can understand Chapleton not getting a 'decent' service given it is pretty much in the middle of nowhere with no dwellings nearby (except a couple of farms and the station house itself and the RGB yard which was probably in days gone by the reason for the station in the first place), but Newton St Cyres I cannot as there are houses quite close to the station, and it's not that far from the village itself (< 1 mile if Google Maps is accurate). That said, even if it had a regular rail service, I'd imagine that the bus service from the village into Exeter would probably be a better shout for anybody wanting that journey as it'd not take that much longer (including the walk to the station) and would take you into the city centre better than the railway can...
This was an interesting read from a regular Tarka line user, against my notes. I made two journeys on a Friday, recently (one to Barnstaple, and one from Barnstaple an hour later - both single 143s too, lucky me
) and both portions were scheduled to call at the same stations. I noted that neither train stopped at Copplestone (0/2), one stopped at Morchard Road (1/2) and two at Yeoford (2/2) and Umberleigh (2/2). It doesn't surprise me that Copplestone would usually have people alighting/joining with the new housing estate alongside, and I did only make two journeys, quite close together, which I can't generalise too much from so thanks for this.
Regarding Newton St Cyres, although the bus link may be popular, it would be good if it is included in a few of the future services to/from/via Okehampton. If it is kept as a request stop then it's not as if the train is going to lose much time if no-one wants to alight or board. This works fairly well on lines like Thornford/Yetminster/Chetnole on the Heart of Wessex line, and the Cambrian coast line whereby every request stop is included in the timetable for trains (bar the Barmouth terminator obviously), but they rarely stop at every station and the timetable factors this in. I understand that the traffic levels for Newton St Cyres, Portsmouth Arms, Lapford, Kings Nympton and Chapelton might be on the light side, and hourly request calls would probably be too much for some (2 hourly would be more appropriate), yet if people don't want to board/leave there, it's not as if the train would lose time through having to stop.
I do however think capacity improvements are needed on this line first and foremost.