First class on FGW is great, SW trains as said is not worth it. When I was at uni travelling back from Egham to Cheltenham I always used to take first class as with an advance ticket it was about £12-14 each way with a railcard which I thought was easily worth it for the FGW leg from Reading.
I'm in Switzerland for a couple of months with work and the trains out of Zurich really put ours to shame

. Not especially cheap unless you have the half fare railcard (£100 or so for a year so a fair whack but does what it says on the tin so easy to make back if travelling a lot) but 2nd class gives most 1st class services a run for their money over here and it's always on time. 1 min late almost seems to be classified as delayed...