If you can travel Off-Peak, it's almost certain a Flexi-Season won't save you anything. especially if you can book an Advance in at least one direction. The target market is rather niche, especially on long-distance flows, and they're almost invariably worse than the Carnet products they've replaced.
Using Bristol - Paddington as an example:
The Anytime Return is £230.00 (£151.80 with a Railcard discount).
If you can leave Temple Meads at 08:30 or later, and return from Paddington after 18:36, you can bring that down to the Off-Peak Return fare of £86.80 (£57.25 with Railcard).
Meanwhile, the former Carnet (10 Returns valid for 3 months) would have been £2070.00 with no Railcard discounts.
The Flexi-Season is £980.90 for 8 days in 28 (so £122.61 a day). No Railcard discount there either.
A Weekly Season is £352.00, for a nominal £70.40 a day (assuming 5 days of use).
A Monthly Season is £1226.20, for a nominal £55.74 a day (assuming 22 days of use, which is the figure used when calculating "void day" refunds under the old Passenger's Charter scheme).
An Annual Season is £12772.00, for a nominal £49.12 a day (assuming 260 days of use, as above).
With Advances, I've just looked for Monday 6th September and there's an £18.15 Advance on the 0800 from Bristol (arrives Paddington 0928) and a £13.45 Advance on the 1902 back (arrives Temple Meads 2035), making a total of just £31.60 Return with your Railcard. Though obviously you'd be restricted to travelling on those specific trains unless you paid extra to change the ticket.
