A-spec is ruined by the seasonals - but they don't add very much to the game.
Out of choice, I have started GT5 again, using only cars with 0 mileage. Too easy to just keep buying cars from the UCD - and then never get to drive them! Got every car on my other account, although you can sort my number of changes, can't easily see how much you've driven any particular standard.
Tend to only play for 20-30 mins - would rather do 3 or 4 races then 1 or 2 online races, so use the seasonals unless I'm almost at the credit limit.
A-spec doesn't add much - few credits and generally not very inspiring prize cars, not easy to set it up to be particularly challenging, once completed not often are they interesting enough to go back to and try to make them more interesting by trying again with lower pp.
Actually, going back to start again, I will avoid many of the A-spec - so that I don't win the prize cars - it isn't like anyone would want them??
At least with the later seasonals, it is interesting to see how little a pp you need to win, and if you fail - still get a reasonable payout, and something like a helmet, paint chip or racing suit (or a penalty of yet another horn!)