My first impression of this game wasn't great. It seemed to be an awful lot of work, for very little reward. But given a bit of time and effort, my impression changed, and I began to enjoy the challenge posed by MotoGP '06.
Ok, it is time consuming, and the achievements don't come thick and fast at any time, but the sense of satisfaction when winning a few of the tougher championships does come.
One tip I would give is to set the laps to 1 per race as this will hurry things up a little, and aim to qualify high, preferrably front row of the grid, to maximise your chances of winning races.
I wouldn't recommend MotoGP '06 to anybody looking for quick gamerscore, but if you like racing games, it is worth buying, especially at the price you will find it at now (I got mine for under £10).
Don't rent though, as again, i emphasise the time required.