The current Waterloo to Exeter service is a superb operation that sums up an awful lot of the good, the bad and the ugly of the last 50 years of the UK railway system.
The Good
Great rolling stock for secondary long-distance operations and very sensibly utilised with 3 or 6 (and a few 9) carriages west of Salisbury and 3 - 10 east of Salisbury. All of the Exeter services are at least 6 east of Salisbury.
The class 158/9 rolling stock is fantastic and very reliable, particularly as it was desiged and built 20 years or so ago. It's better than any of the later generation of diesel units with a quality interior and end doors. The first class seating really is Inter City standard; I would take the option if there are some really cheap advanced tickets available. Standard class seating is the original as-built product and has deep comfortable cushions. I don't mind the FGW HST seats, they are very good considering the 84 seats per carriage which are required to meet demand, but I prefer the SWT 158/9 ones. There are also plenty of table seats in the layout.
The splitting and joining at Salisbury enhances the service, when an eastbound service adds carriages at Salisbury those passengers joining at Salisbury are already seated in the section waiting in the platform at Salisbury thus avoiding a stampede for spare seats. The joining and splitting takes less than 1 minute (DfT please note). The end gangways enable the joining and splitting to be cost-effective (ticket checking, rolling stock utilisation and trolley access) as well as beneficial for passengers. There are the underfloor engines and trolley catering only, but we can't have everything.
The units are also lightweight and, compared to newer stock, have a sensible size of engine and tick the low carbon box. As a template for long-distance stock whether DMU, electric, hauled (!) or alternate to IEP, the 158/9 bodyshell can't be beaten by anything seen since (perhaps the 444 gives it a run). Just think of the economies if all new long-distance stock with end doors followed this blueprint.
The service is now a regular hourly pattern to Exeter and timekeeping is excellent considering the single track sections. Passenger number are very high and increasing - there are a few stock movements that appear empty at the western end of the route, but remember the 6 carriages were probably heaving when heading west into Dorset earlier. The 1820 off Waterloo between Clapham and Hampshire is the busiest service I have experienced recently on any route, and is still very busy as 6 carriages west of Salisbury.
Fares are really good value. Whenever I visit London from Exeter with the family we travel from Pinhoe at just £97 off-peak (essentially open) return for 2 adults and a child on a family railcard. It can be even cheaper with advances. I might even start using it for business from Honiton (Pinhoe is only every 2 hours) since Waterloo gives better access to the business districts.
Signalling renewal is nearly complete between Basingstoke and Exeter (Salisbury remains an island operation). A great example of how efficient in terms of operations (all controlled from Basingstoke) and maintenance (all LED signal heads) modern installations can be. Compare the O&M cost to the previous numerous small boxes on the route.
The Bad
Reducing the route (and others) to a single track for most of the route west of Salisbury was a huge mistake. It was meant to be all about reducing costs, but the saving should have come some years later through modern signalling (see above) and stripping out sidings and crossovers etc.
Just how many routes that were closed or singled would be viable today if modernised with flexible rolling stock, LED signalling controlled from a single centre and ticket vending machines? I think we'd rather have TVMs than a route closure and they are great for pre-booked ticket collection by today's internet generation. Consider the question of viable routes in conjunction with the rapidly falling car ownership in younger generations - totally unaffordable (without mum and dad help) on the average wage of a 20 year-old today.
Fares on many other similar routes are extortionate; we really could do with some consistency to UK fares. An open Exeter to Birmingham fare is something like £150 for one adult!
The Ugly
The single tracking means that fast trains cannot be added to the timetable. Exeter to Waterloo in 2.5 hours would be easy if the route was double track throughout. Linespeed could also be increased to 100mph for long sections.
Why have we procured so much poor rolling stock not suited to the markets they serve (Voyager, 185 .... IEP!) since the 158/9? Believe me, Cross Country would be way better if it had 100mph class 159 type gangwayed stock that would have cost a lot less than Voyagers. You could have run half-hourly to Plymouth with 8 carriages north of Bristol and 4 south (8 at peaks), with similar splits elsewhere. EMT has another good example of this in practice with Norwich - Nottingham - Liverpool.
Whilst electrification is becoming popular (the concept at least), will it really happen for the likes of Basingstoke to Exeter? It should be a no-brainer. The line is a real success story and is here to stay, we should do what any other country would do and invest in its long-term future. Operationally the route could continue as present with the splitting at Salisbury and the released 158/9 stock would be ideal for Cardiff - Portsmouth and a joined up Cardiff - Taunton - Exeter - Paignton / Plymouth / Penzance hourly service (joining up of existing services with rolling stock fit for length of journey).
Summary
Use it, you won't be disappointed and by splitting your booking (separate fare west of Exeter), you will get excellent value for both portions of your journey.
Finally, I should add that the FGW Exeter to London service is also very good. Shame about the density of standard class seating, but it really is needed. Many eastbound services are crammed on leaving Taunton, Exeter or even Plymouth and that's with a half-hourly service in the peaks (1600 - 1900ish from Paddington). The restaurant service available on 4 services per day is also top class. Again replacement of the excellent HSTs should come through electrification without any of this bi-mode IEP DfT rubbish. It's nonsense to say that demand on the route does not justify investment in the infrastructure - in any other country it would have happened years ago. Finally the idea I have often read about that London to the south west traffic could all be diverted via Bristol is absolute nonsense. Do the people who suggest this realise how busy the Berks & Hants services are (a few more passengers than the empty seats on a Bristol service!)? Or would they care to explain how you'd thread a fast service originating in Penzance (always the prospect of delays from further west) through the stop-start traffic on the Reading - Swindon - Bristol route? If we need HS2, we need B&H.
All you need to know about getting from London to Exeter, now and, hopefully, in the future....