Hello there. It's nice there's a GWR Class 387 thread. I'll be honest, apart from unreliable working toilets I can't think of a single thing wrong with the Class 165 and 166 Turbos!
However the 387's are a huge step up !
I'll be very honest, until the reservation issue has been sorted, the 387's are the more relaxing way to get to and from Didcot and London - because of the following...
1. Mainly 8 coaches all day (not bad 4 if a train fault which the chances of this are quite low)!
2. No seat reservations required like on Chiltern Railways services to and from Oxford and London Marylebone.
3. At present, all 387's start at Didcot and Reading when outside of London.
4. Plenty of table seats.
I love them in the summer as twice when I've stepped onboard, it feels like a cool fridge !
When the bus rally in June a few months back took place in Reading instead of Didcot on Sunday 3rd, I deliberately caught 2 Class 387's - the 10:50 from Didcot to Reading and 15:14 back as the times suited me. Both ran pretty much on time.
Plus given the problems on the Bristol corridor, I had more trust in the stopping services than potentially packed IET's!
On any day between the 1st and 5th next month, I'll be heading down to London - a 387 leaving Didcot around 09:10 and 09:40 to Ealing Broadway and returning on the sole fast 387 service from Paddington, 16:36 (calls at Reading and Didcot) which I'm told by GWR in a tweet is an Off-Peak service and is scheduled to be an 8-coach Class 387. I'll have an Off-Peak Day Travlecard.
The '387' services may take longer but handy I'd you need to work or want to have a more relaxing journey. Avoid the Bristol / South Wales services just before / after the 16:36 - not worth the aggro over seat reservations!