I would strongly disagree about value for money, i personally think they have proven far more cost efficient than a manned mission ever could.

There is no doubt that a manned mission would be ideal but you have to admit that having a mission originally scheduled to last 3 months pushing on into its 4th year is only feasible with unmanned craft to do the exploring. It would be prohibitively expensive to sustain a group of astronauts on Mars for the length of time the rovers have been roving.

IMHO robot explorers are the fastest and cheapest way of exploring our solar system. Only if they find something spectacular should the risk and expense of a manned mission be attempted.